The difference is that most people don't think about learning or try to get better at it, because it's a "meta-skill". If you learn the piano, you think about piano. If you learn React, you think about that. Most people don't think about the process of learning itself that went into both of those, so they don't particularly get better at that learning process.

If you do, you'll get better at it as you practice. You'll ask yourself questions like, "What sorts of materials made it really easy for me to grasp the last topic?" If you know the answer, you'll search those materials out.

You'll ask questions like, "What's the best order to introduce concepts in, when picking up a new topic?" If you know the answer, you'll learn the next topic in that conceptual order.

You'll also make the process of learning a habit, just like you would piano practice. The goal isn't to just be awesome at it, the goal is to do it all the time. Being good at it is in the practicing.

Above all, if you inspect your own knowledge you'll find that human knowledge is somewhat tree like. You get a base in primary school. You grow major branches off of that (chemistry, physics). You then grow tiny twigs off of that. And some of us have richly developed and finely wrought tiny branches off of other branches. But the point of "knowledge as a tree" is that to learn something you have to have something to hang it off of. There is no branch without a trunk, so don't try to learn calculus before you can add. There is no tiny branch without a big fat heavy one holding it (so don't try to learn organic chemistry before you know what a covalent reaction is).

If you know the structure of the tree, then you have a map of how to proceed.

To the final question: by reading or by doing? Depends on what you're learning, and depends on who you are, so you need to know a little bit about yourself. Some topics can only be learned by doing (playing the piano). Strangely, some cannot be done without extensive reading! (How to craft poetry, or do research).

School vs. learning by yourself is just a personal choice, dealing with your motivations, having nothing to do with "what's best". It's like rice vs. pasta -- which should you eat for lunch? Bogus question.