Development with thousands of developers is hard. The problems with monorepo and polyrepo are only subtly different. Either way you need a fairly large team just to handle the tools you need to solve your problems. Some of your problems will be because of repo organization (again, both choices have downsides that you need custom tooling to solve).
Note that most of your problems will be related to having a thousands of developers and repo organization is irrelevant.
Note that most of your problems will be related to having a thousands of developers and repo organization is irrelevant.