People have built working operating systems from both the XP/Server 2003 and NT 4 leaks. Here's someone building Server 2003 on Windows 11: https://www.youtube.com/watch?v=AWZe00v2Rs0
I guess they are much better than I am. I remember working on a team at Microsoft where the product took 8 hours to build, and on my own dev machine I made several attempts but never got the build to finish successfully. I worked around the problem by taking a known good build from the build master and integrating the part I was working on into that build. But the experience left me thinking "good luck with that" for building more complex products from scratch.