Answers for "How do I make Python 3.9 default on Mac?"

use python3 as default mac

ln -s -f /usr/local/bin/python3.7 /usr/local/bin/python

set python3 as default mac

# it is said not to do but I used it anyway, because of the conveniency
$ echo "alias python=/usr/local/bin/python3.7" >> ~/.zshrc

set python 3 as default mac

[ -e ~/.profile ] && . ~/.profile