how to get mojave over catalina

Your question as phrased does not make much sense. Part of the Mojave desert is below sea level and Catalina is an island and above sea level, so Catalina is over Mojave. Geography lesson done. Do you have a Mac question?

If you wish to downgrade, you need to erase your entire hard drive and then reinstall the OS using recovery mode. Make a backup of your files first as everything will be wiped out.

He may mean how to upgrade to Mojave ilo Catalina from an earlier OS.

https://apps.apple.com/us/app/macos-mojave/id1398502828?mt=12