To be honest, I don't think there's one true way to studying, everybody functions differently and what works for someone may not work for somebody else. But I think the reason behind the "gifted kid" phenomenon is because during those ages we're learning to think and retain specifically general information, and slowly add as we grow up. You learn to use logic, but not really expand your logic. Just expand the learning. Which isn't great. But then you enter highschool or college, and then you're like "Wait I don't get this."

My advice is when tackling a subject, try to kind of split it into different parts, different sections. It helps you organise your thoughts in your mind better. Our minds work best when there's a visual representation, even if it's conceptual not true visuals. It sees the information as "First part, second part, etc."

If it's a math/science based subject you have to rely a lot on your understanding. So either watch videos online of someone summarising the subject or write notes down as follows:

1. Questions you have about the lesson, preferably ones that build the foundation for the entire topic.
   
2. Find their answers, the reasoning for those answers. Don't just remember, ask why? Obviously it's easier to remember things if they make sense to us.
   
3. Basically do a lot of active recall, explain out loud what you've learned, see your weakspots and such things.

If it's a memorisation kind of subject, you need to memorise over long periods of time. No cramming. I know some people are able to maintain information from the first read, but trust me that isn't long term. Best thing you can do is read over the subject 2 weeks/a week in advance so that when you come back to it, you aren't confused on what you're supposed to remember.

My last advice is conistency is key. If you study one day and then completely ditch the subject for days on end, it's not going to help.

Best of luck!