Comment by fallingsquirrel

I don't know where you got the idea that MS doesn't have copies of their own source code. They aren't just blindly shipping binaries from decades ago with unknown provenance. They keep meticulous archives, and even employ full-time digital archivists. Some of their source has leaked in fact*, spanning all the way from the 1980s to the 2020s, so you can see for yourself if you really want to. If you can download a torrent, you can bet MS has a copy too. And if you search on YT, you'll find people who have built working copies of the OS with these leaks. It's all there.

* https://www.betaarchive.com/forum/viewtopic.php?t=41873