I had spent 29 years at Microsoft before they finally laid me off. I spent that entire time in the Operating Systems division. I wish I could try to describe the sheer enormity of a project like this, but I can’t. We had too many moving parts; I doubt anyone understood everything. We built our product from source code at a particular snapshot in time. To put it in perspective, hundreds of engineers may make changes within seconds of each other, and it all had to work.
Some of these engineers had the unenviable task of coordinating these changes from different places. We called this process reverse integration. The best way to describe it is like air traffic control for software changes. They took a snapshot at a specific time, created a distinct version of our product, and then we ran a battery of tests. If the tests passed, they allowed those changes through.