Why do some built-in Python functions only have pass?
PyCharm is lying to you. The source code you're looking at is a fake that PyCharm has created. PyCharm knows what functions should be there, and it can guess at their signatures using the function docstrings, but it has no idea what the function bodies should look like.
If you want to see the real source code, you can look at it in the official Github repository in Modules/mathmodule.c. A lot of the functions in there are macro-generated thin wrappers around C functions from math.h, but there's also a bunch of manually-written code to handle things like inconsistent or insufficient standard library implementations, functions with no math.h equivalent, and customization hooks like __ceil__.