> How does a “small startup” end up with a multi million line “legacy” codebase?

Easy! The output of 6 months ago Opus! Which seemed so wonderful at the time.

Have you worked at many startups?

You'd be surprised. I met a guy last week who was proud to tell me he had vibe coded an almost 2 million line code base. The app did not sound that complicated, so I'm assuming it's full of copy-pasta flavored slop.