I’m a software engineer, for stuff that are related to library/programming languages, I usually try to look at the official documentation first. Some of the best documentation pages IMO are: SQLite, PostgreSQL, and Rust book. MDN is good too, like you said. Looking at the source code helps too, especially the test files. I currently write lots of Go (programming language) code, and IMO their test files are so good, and you can learn how some functions behave or how to use it.
Other than that, I actually starting to learn to draw too, and for that I, so far, use drawabox. I think it’s a good starter for learning to draw–as it gives you the most fundemental stuff when drawing. (technically not online, rather a book, but just before opening this thread I found this resource for learning drawing perspective: …archive.org/…/Norling - Perspective Made Easy.pd…)
I also wanted to start playing guitar, but haven’t got the money to buy one, but from what I hear Justin Guitar is a good free online resource. Haven’t used it though