"you think they let MS employees just commit to any codebase they like?" That's pretty much how it works at Google. The code owners have to review and approve

"you think they let MS employees just commit to any codebase they like?" That's pretty much how it works at Google. The code owners have to review and approve your change (and can reject it), but it's common (even encouraged) to commit a bugfix or new feature to someone else's project.