Jonathan Goble writes:

 > I assume the "cpython" part of these paths here is your local clone of the
 > CPython GitHub repo? (Otherwise these local filepaths from your computer
 > don't make sense.)

Thanks for catching that!

Sorry, I was concentrating on stifling irrelevant Unicode
politics. :-)  You need a local clone of the GitHub repo, and the
various possibly relevant files are in

Doc/library/codecs.rst  (these two are available online)
Doc/c-api/codec.rst

Lib/codecs.py

Python/codecs.c
Include/codecs.h
Objects/stringlib/codecs.h
Objects/unicodectype.c
Modules/_codecsmodule.c
Modules/cjkcodecs
Modules/clinic/_codecsmodule.c.h