One thing I don't understand about monorepos is that do people just stay on one platform and check in binaries? Or is it assumed that everything must be compile

One thing I don't understand about monorepos is that do people just stay on one platform and check in binaries? Or is it assumed that everything must be compiled and correct. I get that a branch can be compiled, tested and integrated, but how does that work with multiple teams. I mean at what point does it become like week-long builds to make sure everything is accurate and correct. Or is monorepo more of a "place to put all the code" not necessarily correct or working. I like multiple repos because it's easier to assume that the main branch of each is "correct and tested and excellent quality".