Yes, that source code is available, and it’s obviously a very large set of collaborating subsystems that make it all happen.
If you check out the source code from GitHub, and make the Visual Studio projects, you can then do Find In Files for some kind Direct3D primitive function, such as IASetInputLayout().
Then you can start following callers up through the call chain (and through indirect data structures) to figure out how all the data gets to where it needs to go.
Find-in-all-files on Windows, and “grep -r” on Unix, are your friends when trying to make sense of a new code base! Start with some terms that you know will have to be used (such as the graphics API) and go from there.