Heh, PRs were actually quite small (from what I saw), and many teams worked on their own repos and then grafted them into the main repo (via subtrees and automated commits). My team worked in the main repo, mostly on framework-ish code. I also remember quite a bit of other automated commits as well (mostly built caches for things that needed to be served sub-ms but changed very infrequently).

And yes, spending two-to-three weeks on getting 200 lines of code absolutely soul-crushingly perfect, sounds about right for that place but that has nothing to do with it being a monolith.