Looks like someone grabbed and posted all Windows source code. I've read there's a few 64-bit ARM builds as well as "Microsoft's Windows 10 Mobile Adaptation Kit, a confidential software toolset to get the operating system running on various portable and mobile devices." Wonder what that means...
https://www.theregister.co.uk/2017/06/23/windows_10_leak/
https://www.theregister.co.uk/2017/06/23/windows_10_leak/