Trial and error, books of various sorts, instructions sheets that come with new parts (even though I am male, I learned to read instructions as a short cut to success
); and now of course the internet, but not as frequently as some, and I haven't ever had the need to watch a YouTube video. Having a fleet of hire and tour bikes for my own small business meant a lot of maintenance work that I did myself.
Some say that to learn, a person should get a beater or crapped out old bike to learn on. I tend to disagree, and there are others who do, too. Old bikes with rusted-on components aren't really going to do you much good in the context of (a) getting them apart because they can destroy good tools and your hands! and (b) the components generally are outdated and not worth the effort (except some good-quality ball-and-race BBs and hubs). Same comments apply but moreso with Walmart and similar big-store bikes.
Get a well-priced, reasonable quality second-hand bike that fits you, adopt it as a back-up, and work on that. You will find it much better.
The exception to this is if you find a good quality older frame that can be stripped of old components, and rebuilt with new ones. My best-riding bike is a Shogun 400 that I picked up at a dump for nothing. I kept the frame, fork, seatpost and BB, and ditched the rest so I could rebuild it with new components as a fixed gear; I've toured Europe on it, and done a year of century-a-month and other rides.
All in my own opinion, of course.