Hi Hermie,
Never there existed a Windows OS that was re-written from scratch. It always has been built with new layers on old layers, and a lot of tweaking has been taken away from end-users. Think of what one could do in the past with your old DOS-malnual in hand? Even their new browser, edge, has lots of old IE code. Remember the days of the New-York hired-in spaghetti coders that had to help Redmond meet some timeline. Propriety code is almost entirely written in C, C++, and C# for Windows. Some areas of code are hand tuned/hand written assembly.
pol