In general my bucket for "learned" means that I have a mental model of the landscape of a problem/fact area and a decent memory of the core sources (books, wikipedia pages, whatever) I used to figure it out. So far I haven't had trouble working backwards to refresh my understanding using those as a guide. And again, if I have managed to forget not only a fact but where I learned it... there's not much in my brain to refresh, is there?
I see the personal wiki as an evolution of the commonplace book, that was kept in the past by the likes of Darwin (https://en.wikipedia.org/wiki/Commonplace_book). Here are some of my notes from the book Where Good Ideas Come From:
"We know much of Darwin's thinking on the development of his ideas from his extensive notebooks, which he read, and re-read and recombined. This era was the time of the 'commonplace' notebook, in which long passages of quotes from other sources and thoughts were recorded. Reading and writing were apparently quite related. These books may have struck a balance between silo'd organization, and utter chaos, allowing the development of theories beginning with hunches which could then be further developed. The key to developing a hunch into a theory appears to be writing it down "
This is a repost from a previous thread regarding ideas , but 'learned information' and 'ideas' are both just types of information to be managed.
show available quickref files (in directory $QR)
$ qr topic
show all lines from $QR/topic.txt
a "topic" can be anything, but generally something like a language (py),
application (blender), package (django), command (git). also things like audio,
pdf manipulation, CLI image editing.
$ qr topic pattern
show all lines from topic.txt that match regex pattern
$ qr topic term1 term2 ...
show all lines from topic.txt that match all terms
$ qr add topic "line with spaces"
append "line with spaces" to topic.txt
$ qr edit [topic1 [topic2 ...]]
open specified topics with $EDITOR
I work my memory out a lot, though. Some days I'll collect license plates, or of I am reminded of something but can't remember where it comes from I'll spend hours or days working to pull that memory back to the surface. I use search engines to gain new information or for confirmation of things I'm not sure of, never ever to help me remember things I feel I should remember but have forgotten.
Reading books and writing some things by hand are both helpful. I have some favorite books that I've read repeatedly and knowing whole passages is valuable. Writing is the best aid to memory. Often I'll make notes on something and then never look at them again because the act of writing (which is slow and tedious compared to typing) has helped to fix them in my memory.
I also start small git repos with projects, e.g. for a set of algorithms I explore.
Haven't found a good system though, a hierarchy or learn graph or mind map (mind map programs kind of bore me).
My hope is that most of what I write down will compile (so to speak) into my brain, at least a little bit. And that the mind map will emerge actually in my mind as I move along.
This system is not perfect, but I haven't started to optimize it (maybe I should).
One next step that I think about is to product short texts in a top-down fashion. That is, start with some high-level idea to express or illustrate. Then descend into special concepts and so on. Writing is to thinking, what walking is to a map.
...which is the best thing I've found for notetaking (I'm the author). It needs simpler installation and added features but is stable and works really well, really efficient once you get familiar, and everything is on the screen. I hope to add anki-like features in the future. Contributions welcome.
Edit: this program is like a textual mind map that is highly efficient to use from the keyboard, uses postgresql, and can handle large amounts of data, having the same thing linked in more than one place, etc etc. I use it to keep lists of gift ideas, todos, calendar, all notes, personal journal, and it just gets the job done with the lowest impedance of anything i have tried or heard of. It has an auto "journal-generation" feature, some finicky import/export features, search, somewhat limited file storage, and more.
EDIT: ps: Anki is a really useful review tool, like flashcards that space things out to months or years depending on how well one knows the material (as perhaps others like it).
I use name metadata for the notes with [broad topic] - [sub topic] - [optional very specific topic] - [optional date]. I'll probably revise this soon, since it doesn't work perfectly.
I wrote an automator service to insert an ISO8601 date, which is bound to cmd-|, and I use it a lot.
+ Work - HPC - Module Installation - PLINK GCTA KING - 2017-07-21
+ The date is for when i started, but there may be sub-note dates for logged work if it happens later (lets say that the install needs to be fixed, or was broken, and I want to annotate both the discovery and the fix). I will probably change to not put dates in the filenames unless they are time-specific notes.
+ Work - Meeting - PERIOD GROUPNAME - 2017-07
+ These ones i keep one note per month, with dated subsections for each meeting, but am considering keeping one per year (or one period). Performance for really long notes is good. Luckily, merging notes is just a cat redirection away (so easy).
+ CLI Notes and Snippets
For regular use I just use the built-in editor (which is ok, but not great, and has some indent issues). For serious notetaking, I use the built-in hotkey to open the file in sublime (which has much better markdown handling). Full text search makes things easy to find. Sometimes I put a keyword block at the top of the note if I think it will be hard to find/remember. I can use grep if I need to do serious searches.
I've been using this method for about a year, and have accumulated 669 note files so far.
There are a bunch of other features in NVAlt that I don't use. The ability to paste readable content from a web URL is pretty cool (though I don't use it often).
For things that involve code, e.g. an actual implementation or particular snippet, I use org-mode. It allows you to embed code as part of a regular document. I keep both thematic documents as well as a file per day for loose notes (the idea being that stuff that I collect will eventually be polished up and make it into thematic notes, but that never happens).
I also have a git-based go-to journal program that I use to write down other things I learn. 
Other than a web browser and a terminal, these are two of my most used programs...
Unfortunately, I wrote this in odd free moments at work and can't just release it without going through a rather convoluted process. It's only about 400 lines of Go, though, so anyone who thinks this sounds kinda neat should be able to replicate it pretty easily.
Oh, it also has commands that let you store a copy of a given file with the current note, so you could e.g. take screenshots as you're making notes and they'll live with the note.
Edit: I've played with org-mode but it's kind of a big beast and I like separating out all my notes into individual files, plus I'm only occasionally an emacs user (I get the bug, use it for a week, then get sick of it and bail out)
Re org-mode: It's easy to split out into multiple files using links. I have an index.org in my ~/org directory that links to many topics, each linking to more (or each other). It's effective for creating a personal wiki-like document graph.
The other way would be taking notes. I'm really bad at managing notes at this time but when I take notes I can recall them for a few months.
But, going back to my previous paragraph due to the fact I learn by doing I remember things that I do repetitively.
Since I moved over to Linux, I settled on ZIM / Desktop Wiki. I love that it's directory/file-based. The organization is done via directories. The content are text files. I no longer have to worry about whether alternateOSx or mobileOSy will support my content. Any filemanager will do... in a pinch.
 screenshots: http://www.tranglos.com/free/keynote_screenshot.html
It was all one big mind map and was quite hard to manage so I decided to split it up and focus on semantically linking things together with mind maps and providing resources for these topics. I also decided to make it into a search engine so all have a chance to edit and fork the content.
Here is the search engine : https://learn-anything.xyz/
I still keep my notes as mind maps, just don't publish them in the search engine :
All of that said, I don't have a good, comprehensive, over-arching approach. It's a bit of a jumble right now. That said, in the name of "eating your own dog food", I will eventually be applying some of the stuff we're working on at Fogbeam Labs, especially in terms of using semantic web tech and machine learning to automatically mine rich semantics from documents and automatically link them, etc.
It may make sense to open up a YouTube channel though, so I can go back and review things later on. Thanks for bringing this up, because I think that's a very actionable goal.
Over the past year, I've been trying to go deeper with my Swift/iOS knowledge and creating small example programs so I have can build a reference as I learn something new:
For a better search, I load the local repo into Sublime.
The markdown files are published to my site...which I accidentally overwrote https://www.aizatto.com/notes/
Google Spreadsheets example:
-  https://www.robinwieruch.de/categories/growth/
it's company-public and i've had others comment on how some of my TILs helped them. i've also had others offer their own insights to what i learned and subsequently learned more.
I also make Anki clozure cards for key facts to try to keep the skeleton of it in memory. Helps quite a bit to keep some points in active memory. I do the reviews daily.
The firefox search bar only works for the titles. Since I wrote them, I usually know what's in them.
I always write them locally anyway, and then push the gist up to the server. Since I wrote them locally, if I really need to search them, I just use "ack" on my directory full of gists.
Often they are a local git repo containing markdown, org-mode and code. The advantage is that it's easily searchable and ages well.
Over time I only keep most the most essential pieces of information.
Works for me, but your mileage might vary
Also check out AnkiApp
It's not an ideal system, but practical. In college, I remember coming across THOUGHTSTICKER, which I would recommend anyone working on e-learning to research and steal ideas from.
Why two separate apps? I find Bear Notes handles Markdown much better which is my preferred method of note taking. On the other hand, Evernote is better at OCR, media, etc.
I hope to one day transition everything to Bear Notes but until there is feature parity, this is my setup.
Something about writing stuff down with a pen on paper commits them more to memory than typing them.
I use a markdown file, called ThingsToRemember.md, with absolutely no organization.
I've started with org-drill recently (been using Anki) to try it out. Not sure if I'll switch, but may use it for topics that are inside of my org files already to help keep my memory fresh on topics.
I've also started experimenting with Incremental Reading  inside org-mode. I've only done this with a couple things. Take an article or ebook or other document and get it into an org-mode structure. Mark each item with READ, UNREAD, and NEXT (to track your place in the document). As I read through it I have a capture template for creating drill notes from a marked region (very basic, just makes an item with a tag of :drill: and the contents of the region). Then when you do your drills during the day, these items will be brought up for practice. When you return to the article or text (in theory, haven't been trying this long) you can pick up where you left off because you've got the important parts of the old, read sections in your memory (and practiced).
I will be trying it with some work documents soon (many spec and requirement docs for a set of systems I don't know well, pull out important details and document names so I at least know where to look for things). An experiment in building up a better understanding of complex systems that I didn't have a hand in developing. In the past, I've relied heavily on SMEs and test engineers, more than I should have, and on this project in particular we have no SMEs for the bulk of the system.