While not open source, I've seen parts of the windows source (never really had the time to read it in full detail) in my work, most probably coming from the MS vs EC situation some years back[0]. Yes, there are still hidden parts (docs mostly, I recall my friends doing memory forensics research had a "fun" time to get information on IP connection structures for example), but it's not as secretive as many people think it is.

[0] https://en.wikipedia.org/wiki/Microsoft_Corp_v_Commission#Si...