This article explains how to reinstall macOS Catalina on a Mac. Apple makes it easy. All you need is an internet connection and you're good to go.

It’s a good idea to back up important files before you reinstall. You’ll need a reliable internet connection, since your Mac will download the files it needs from the internet. Apple built a recovery mode into macOS for those times when you need to reinstall an operating system.

Click the Apple logo and select 'Restart'.

As your computer begins to shut down, press and hold Command+R. Continue holding these keys past the black screen and/or the music you hear as your computer boots (depending on how old your Mac is) until you see Recovery Mode. Then release the keys.