I've seen usages for PyId_ variables all around the CPython source code. I couldn't trace back to the definition of any of them.
Examples include PyId___name__ and others.
Who or what resolves them into strings? I've seen a C-analyzer with known.csv file on the CPython source, listing most of them (but not all). Not sure if it's relevant.
As @jasonharper said, PyId_ variables are defined using _Py_IDENTIFIER, usually found at the top of the source files.
Snippet:
#define _Py_IDENTIFIER(varname) _Py_static_string(PyId_##varname, #varname)
_Py_IDENTIFIER works using stringizing and token-parsing operators which allow you to define new variables programmatically.
The string values are then interned for better memory efficiency.