terminal says "command not found" when I type in sudo /Applications/Install\ macOS\ Catalina.app/Contents/Resources/createinstallmedia --volume /Volumes/OldAirOS

You need to download the macOS installer first before running the create media command.

To add to @T3sss' post, sometimes macOS will just download a stub installer instead of the full macOS installer. Plus sometimes when this happens, even if the full macOS installer is downloaded, the installer will be broken in two and stored at two different locations (who knows why). Check to make sure you have the full macOS installer in the Applications folder. Usually it should be about 6GB in size. If you don't have the full installer in the Applications folder, then the "createinstallmedia" utility won't be there either. Trying to get macOS to download the full installer to the Applications folder when this happens can be difficult and usually will result in needing to use the command line.

That command createinstallmedia does not work on all versions of the OS. If you need to create a Catalina installer, download the installer from How to get old versions of macOS - Apple Support

and use

https://diskmakerx.com/

to create the disk.