-bash: emacs: command not found (after updating mac to CATALINA)

As of Catalina, Emacs is no longer provided in macOS. You'll have to download it from elsewhere, such as https://emacsforosx.com/ or https://aquamacs.org. There are other packages/distributions/ports listed at https://www.emacswiki.org/emacs/EmacsForMacOS.

If you have one of those packages, you may have to re-install its command line tools/helpers.

If you can live with basic "emacs-like" editing, Catalina includes "mg" text editor. For more info:

man mg