I don't disagree that having a good cross-reference/indexing setup is wonderful. I'm fortunate in that most of the projects I've worked on are sensible enough that I can at least narrow down the couple of locations it might be in, and where it's not so easy, it's at least something where "oh yeah, all the interesting code in LLDB is actually in Plugins/" can easily be slotted in the brain. (glibc is another horrible one)
One of the things I've considered doing is putting together a recording of fixing a bug in a project whose source I've never seen before, as the only way I can think of to explain how to navigate a large code base without accidentally skipping several steps of "I apparently memorized the layout of the monorepo without realizing it". This plan is defeated by... not having a code base I can use for this where I haven't actually poked around its source before at least once.