Start here:
/Users/steve/src/Python/cpython/Doc/library/codecs.rst
/Users/steve/src/Python/cpython/Doc/c-api/codec.rst

To write them in C, follow the code in
Likely needed (forgot where the Unicode codecs live, try codecs.[ch] first):
/Users/steve/src/Python/cpython/Python/codecs.c
/Users/steve/src/Python/cpython/Include/codecs.h
/Users/steve/src/Python/cpython/Objects/stringlib/codecs.h
/Users/steve/src/Python/cpython/Objects/unicodectype.c
/Users/steve/src/Python/cpython/Lib/codecs.py
/Users/steve/src/Python/cpython/Modules/_codecsmodule.c
Probably not needed:
/Users/steve/src/Python/cpython/Modules/cjkcodecs
/Users/steve/src/Python/cpython/Modules/clinic/_codecsmodule.c.h