It's not only Windows that uses Git at Microsoft, but Sharepoint and Office (which includes the on-prem version of SharePoint). In terms of repo size Windows and Office are similar. I was part of the team that migrated Sharepoint from a Perforce clone to Git and helped build the tooling to allow Office to move as well. VFS for Git [1] and Scalar [2] are really good pieces of software.
I hope one of these "large file/many files" extensions becomes part of git core eventually. They are too much of a hassle to deal with and many servers don't even support them.
[1] - https://github.com/microsoft/VFSForGit
[2] - https://github.com/microsoft/scalar