With Python 3.9 on Macintosh Big Sur, you can copy the BirdBrain.py file into:

/Library/Frameworks/Python.framework/Versions/3.9/lib/python3.9/site-packages

This will make the module available to all python programs.

You can also use a symbolic link to point to a more accessible location, such as:

cd /Library/Frameworks/Python.framework/Versions/3.9/lib/python3.9/site-packages
ln -s /Users/fmorton/GitHub/neighborhood_makers_birdbrain/lib/BirdBrain.py

Also, if you want to set the initial directory IDLE uses when opening a file, modify the file:

/Applications/Python 3.9/IDLE.app/Contents/Resources

by changing idlemain.py to look like this in the top part of the file:

# Change the current directory the user's home directory, that way we'll get
# a more useful default location in the open/save dialogs.
os.chdir(os.path.expanduser('~/GitHub/neighborhood_makers_birdbrain'))

WARNING: If you change this directory and at some point that directory gets deleted, IDLE will not startup and will not provide any sort of error message.