How and what exactly should I search on YT or EdX? Just "music theory" will do? Also, it'd be amazing if you could point to me which book would be most appropriate to start with: https://www.amazon.com/Glory-St.-Germain/e/B00J4ZOF6W :D
The white book is a compressed version of the previous three and is much faster paced. It's meant for people with some musical experience and exposure to theory. That said, it also doesn't spend a huge amount of time on any one thing, so while it has all the same basic information as the other 3 books, it spends less time exploring each concept.
The green and purple books are more for when you want to get your theory certification from some authority (like RCM) and are more advanced :)
https://ultimatemusictheory.com/ also has some online courses and info available.
But yes, searching for "music theory" and things like the circle of fifths, intervals, modes, scales, pitch, standard notation, and keys would likely turn up useful results. Mostly they'll assume you have an instrument (usually a piano) handy.