User blog comment:P進大好きbot/Elementary Large Number beyond PTO(Π11-CA0)/@comment-34422464-20190603175154/@comment-24923514-20190603233552

He probably didn't take many (if any at all) classes explicitly on proofs. You kinda just learn how to do it by being exposed to all kinds of maths and challenging yourself to create elegant precise arguments. (Not that I can do any of that necessarily; that's just what I've heard proper mathematicians say lol)