to commonly asked questions.

Can I still download Catalina?

Can I still get Catalina? I can't find it on the App Store.

Yes. Apple has a stupid policy of removing public access to old versions of the operating system when they release a new one. This is ridiculous as there are many reasons people could still use a link to old versions. Say you have a computer that is too old to update to the latest version but you can't find its last supported OS. Even using recovery mode, that only takes you back to the OS the computer shipped with then you're just back to the same situation. Bad choice by Apple. Good news though. They've not taken old OS access down completely, only removed it from public view unless you know where to look. Here's a direct link to Catalina, still available (for now) in the Mac App Store. It's free.
This image is a theme.plist hack