Where I can learn Mathematics behind Computer Science?

Answer 1

Where exactly do you start your journey? All mathematics are part of computer science too. There are no topics, which would not also concern us. Only few are too specific in some areas, but in general there's nothing to exclude. And the question where to learn all (!) mathematics would be a bunch of books. Digital, at school, university wherever.

Answer 2

It depends how you learn best. I read through a calculus textbook and did all the problems to learn calculus, but some people do well with videos or in-person teachers. Find a curriculum and stick to it