https://www.microsoft.com/en-us/sharedsource/default.aspx
https://www.zdnet.com/article/microsoft-opens-source-code-to...
A lot of Microsoft's code has also been leaked:
https://www.theregister.co.uk/AMP/2017/06/23/windows_10_leak...
I don't know what the situation is with iOS. How much is in common with Darwin?