I work at Deloitte. Every time I chat with my counterparts in Tax, Audit, Wealth, ... I come up with ideas that may or may not be worth pursuing further. After that, it's just a matter of seeking out vicious rejection of all the bad ideas, till you land on one where the best objection is "well, we just don't do things that way around here".
There's also just being the guy in the right place at the right time, and being open to learning new stuff. I ran into a guy ~20 years ago who'd found a niche in building software emulators for ATMs. He didn't have a coding background, but after asking lots of questions of his buddies who worked in banking and taking a few night classes on Visual Basic coding he'd built a multi-million dollar business on his own to fill a gap that almost nobody knew existed. Thousands of better coders working in banking could've done what he did, but none thought to even look at the opportunity that was staring them in the face