Where can I view Unity's own code and definitions?

I’m trying to get a better understanding of how everything runs behind the scenes with Unity, and to do this I want to see how certain things are implemented, like GameObject etc. If I right click GameObject in VS and go on Go To Definition, I can get to the defining header but can’t get manage to find where everything is actually done. Is there any way for me to look at this?
Thanks in advance!

Unity is not an open sourced project. Only enterprise users have access to source code.

Unity itself has some repositories which might help you to understand how things works: https://docs.unity3d.com/Manual/OpenSourceRepositories.html

However, there is also this unofficial repository: GitHub - MattRix/UnityDecompiled: Now unnecessary, use the official code instead: https://github.com/Unity-Technologies/UnityCsReference
It will give you all your answers, but I guess you could have found that with a basic google search.