The absolute best thing I ever did in regards to figuring out bike maintenance was to buy a really crappy bike and just try to fix it, similar to what you’ve done. I went into it with the attitude of “if I break stuff, that’s fine, it was super cheap and old anyways” and wasn’t imagining I’d actually get a sound bike out of it. I used park tool YouTube videos mostly, and from that bike (and a few others) I learned how to do pretty much everything maintenance-related short of redoing the seals in a mountain bike fork (and that’s likely coming up soon). Wheel truing is tough but absolutely doable - again, but a really cheap bike (marketplace special), take the wheels off and apart, and just try to get them back together - that’ll force you to true them. Park tool again was an awesome resource for that.