Make notes and use "chunking" to group similar ideas into groups and build up from those. It's easier to learn a bunch of small pieces and combine them, than it is to attack an entire subject all at once.
Use multiple sources and YouTube extensively (lectures/enthusiasts explaining difficult concepts). Sometimes a slight tweak to how a topic is presented, along with the context from past passes in other sources is enough to improve understanding.
Work through problem sets and free-recall to avoid cognitive biases of thinking you know some material better than you actually do.
Spaced repetition systems like Anki.
Experiment with all of these (and other approaches) and be objective about which work best for you.
Some helpful resources I've found over the years are:
Several other comments on this thread mention similar methodologies, as well.