How to learn about anything

In my humble opinion learning about anything is about reading and having some type of comprehensive knowledge. By comprehensive knowledge I mean knowing some nuance and deep knowledge about the subject matter. Something that school has a hard time teaching you. For instance when I was first starting to learn the guitar I was in a guitar class. I was taught how to play chords and a couple of songs and was tested on it. Any musician who actually plays a ton would think this is silly simply because your knowledge grows with an actual interest not by simply playing a couple of songs.