If you really want to see some Windows source code. You can just ask them - and they will send it to you. It's not open source and there are limits to what you can do with it.
They call it their Shared Source Initiative. They want a reason for sharing it with you but I have used, 'I am just curious.' With that excuse, this was a long time ago when I still used Windows, I got the specific code I wanted for Outlook Express.
I haven't worked directly with Microsoft for well over 15 years, but this sounds similar to what I remember. Back then I worked for a partner who was doing direct integration work against low-level SQL Server and Windows libraries. Often when we encountered obscure bugs, they'd just give us the SQL Server or Windows source code and basically say, "Fix it, and we will release a hot fix." All of the comments would be replaced with white space which made things more difficult.
