How did Einstein come up with general relatively?

General relativity has always fascinated me. It feels so much out of the box, so absurd and yet so beautiful. No wonder it was so much controversial during Einstein's time. The man and his magnum opus were a hundred years ahead of their time.

I'm currently a undergrad college student, right now I lack the mathematical knowledge to fully grasp general relatively, hopefully in a few years of hard work I will be able to fully grasp it.