I don't know much about Windows development, but I'm sure the system is modularized in some way. Why wouldn't you want to break up the project into multiple repos for different parts of the system? That would let you work on and test each part independent of the rest. Each part should be able to function on its own, right? Of course some engineers would need to build and test the entire OS as a whole, but I'd wager that (for example) the team working on visual design of the settings app doesn't need to have the source code of how the login screen verifies passwords.
Clearly Microsoft's process works well enough for them, so I wonder what benefits there are to using the monolithic repo choice over many smaller repos.
Google has a single repo. The advantages are that you don't need to version anything because you always build against head. It's awesome but requires some discipline and good infrastructure.
If you have many small repos for a large interconnected project you simply move the complexity of managing a commit that requires changes into another tool that can manage cross repo changes and dependencies. With a single repo you can change something and build it, fix any breaks and then commit it with just source source control and build system. The many small repos has in my experience been driven by either poor processes or tooling limitations.
Clearly Microsoft's process works well enough for them, so I wonder what benefits there are to using the monolithic repo choice over many smaller repos.