>How were these people able to transition from founders of a tiny company to successfully running and managing large (people or revenue) companies? Many of them had never done this before so how did they figure out what to do and how to do it?

lots of mentorship from investors and others with experience. Even the most famous young founders had a veteran executive running a lot of things behind the scenes. Steve Jobs had Mike Markkula, Eric Schmidt was running the show at Google, Facebook had Thiel and Sean Parker. This applies to pretty much every one of these companies

most of the startup myths about these companies are embellished because having a young prodigy founder is good for marketing. Of course some are never able to make the jump and get replaced by the board