For example, there are features of a topic on the w3schools site, and there are features that are not used much on the reference page. Do I need to know all sub-titles?
I’m not sure that I understand the question.
If you’re asking whether you are expected to have every feature of the language memorized, then the answer is no.
Because it’s a box bigger on the inside than the out. No matter how much i learn, there is always more. Not that you can’t learn it all, but is that a reasonable goal? Most of the time, it’s learn the basics and research the rest. Keep bookmarks of the things you’re curious about, and references
My first five bookmarks in all my browsers: