I think maybe the best question to ask in response to yours is: why? Is there something you think should be improved? Is there some question you have about how it works or how it should be used or why you are getting some result instead of the result you’re expecting? Do you want to use the source code as a sample to help you learn C?
If you could tell us your goal, we would be better able to help you get there.
RobR
P.S. I’ve been a C/C++/C# programmer for well over 20 years, and I doubt that I could understand the PostgreSQL source code. But I’ve never been tempted to look at it.