Occasionally, we may still need emacs for small editing, but on Catalina or Big Sur, emacs is not available. There are some options: one is to use nano or vi instead. Another is to install emacs using Homebrew. The third option which can be simple is just to use mg. It is MicroGnuEmacs and when you use it, you can use the familiar keystrokes as in emacs.

If you are used to typing e for emacs and using emacs for git, simply change the zsh alias in ~/.zshrc to

alias e='mg'

and config git:

git config --global core.editor mg