looking fo documentation for module files

[snippsat]

why not a /usr/site directory?

You can use what folder you want,look at Gribouillis answer in post #6 or my post here.
If i do quick run on older Kali Linux that i where i use default Python and pyenv.

# make folder that want to use 
root@kali-tom:~$ mkdir /home/our_modules

# make a <sitecustomize.py> in our Python version
root@kali-tom:/usr/lib/python3.7$ vim sitecustomize.py

# Add in sitecustomize.py
import site

site.addsitedir('/home/our_modules')

# Test that it work
root@kali-tom:~# python3
Python 3.7.5 (default, Oct 27 2019, 15:43:29) 
[GCC 9.2.1 20191022] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import sys
>>> 
>>> sys.path
[ '/usr/lib/python3.7', '/usr/lib/python3.7/lib-dynload', '/usr/local/lib/python3.7/dist-packages',
'/usr/lib/python3/dist-packages', '/home/our_modules']
>>> exit()

i generally target 3.5 and up, with my public code.

Python 3.5 can say is dead💀 as many modules/libraries dos not support for it anymore.
A core module for many other that many has dependencies of is Numpy.

The Python versions supported by this release 3.8-3.10. Python 3.11 will be supported when it reaches the rc stage.