I was looking online for the best place to ask this question, as it seems there is no good answer anywhere. I am really sorry if this is not the right place to ask this, so if that's the case, I would appreciate if you could refer me to the right place. Here goes.
I am a junior in high school who is very passionate and curious about mathematics. I have participated in numerous math competitions (such as AMC, AIME, and USAMO), and I have completed all high school math courses up to multivariable calculus. I also developed quite a lot of experience writing rigorous mathematical proofs. In my free time, I love exploring math by coming up with problems of my own and attempting to solve them (or at least trying to make some progress). After a while, I would search it up online, and oftentimes it turns out that some mathematician has already answered it. However, the methods used to solve those problems end up being beyond the scope of my knowledge. This leads me to my question: how does a high school student like me start learning higher level mathematics, such as Analysis or Number Theory?
I know it might be a bit early for me, since I am just in high school and most of you will tell me to wait until college, but I really want to start learning now. I am deeply curious about pure mathematics, and I want to build up knowledge to a level where I can eventually get involved in mathematical research.
More specifically, here are the two questions I seek an answer for:
How does a high school student start learning higher level mathematics? Specifically, I am curious about topics like Analysis and Number Theory, mostly because I already have a strong foundation and experience in them (mostly from doing lots and lots of questions from math competitions). I am looking for any specific textbooks or resources that you might recommend.
More generally, how does one make a transition from high school mathematics to a level where one can actually get involved in mathematical research? I've been looking at famous mathematicians who achieved all these crazy things, and I've been wondering, how did they reach the level where they were at?
Sorry for the soft question, and thank you in advance for your responses.