How to be enough smart to be a mathematician?

Find a good resource with lots of difficult problems to solve. The resource is basically useless if it doesn't come with a well written answer key.

Leetcode is good for learning leetcode. The Art of Problem Solving books are good for pure math. Stuff like that.

That'll get you through the "basics" if you really like math and also go to undergrad/grad school for it. If you want to be "the best" then you'll have to compete with people who actually are geniuses. Idk how to compete with them, maybe read their blogs and try to replicate their practice methods?

Here's a really smart guy's blog: https://terrytao.wordpress.com/

What a strange question. Might as well ask how a fish learned to swim or a bird to fly.

In all seriousness the biggest influence likely comes from friendly and enthusiastic teachers and a then all the things necessary to support having enough time to sit and think. Strong and true autodidacts are few and far between in the general population. And even then it's a very rare one that can not only describe their internal state let alone allow its transmission to others.