Unity render pipeline source is available here: https://github.com/Unity-Technologies/Graphics

All the C# code running in the editor and runtimes is here: https://github.com/Unity-Technologies/UnityCsReference

The code that interfaces directly with the platform API at the C++ level is restricted (you can get access but it's not really a viable option for a beginning graphics programmer :)). For many platforms the APIs themselves are proprietary and therefore that code cannot be shared easily.

There's pretty good tooling support around graphics debugging: https://docs.unity3d.com/Manual/RenderDocIntegration.html https://docs.unity3d.com/Manual/FrameDebugger.html