Anything and everything imaginable, you can learn about it on YouTube. There are step by step visual instructions on how to build stuff or take stuff apart, or even if you wanted to; learn about something completely new.
Instruction manuals that are given with a product are from some corporate company where these videos are just from regular people. The videos also show better visual instructions than just small non-moving panels with captions do.
There are also videos out there of stuff you can do that there aren’t any instruction manuals written for. Some of it illegal, others just cool things you can do with products already available to the public. Since its Valentine’s Day, here is a cool video on how to make a rose.