well... I don't know how helpful this is but.....usually you can't go back once you upgrade.
Sierra is, I think, 10.12, High Sierra is 10.13 and Mojave is 10.14
El Capitan is fine, being a bit more advanced than Sierra/High Sierra, but not quite as demanding as Catalina
Starting from Sierra/High Sierra onwards, the OS installer will check for a Metal compatible video card, install the appropriate firmware, then start with the installation of the OS. Thus your Mac will have to restart twice, once for the check, etc, then at the end after the OS has installed.
If you have Sierra/High Sierra or El Capitan, you can definitely upgrade to Mojave. Usually, though, Apple does not keep copies of its older OS'es around. You can get them again if you "bought" them before from the App store, and I'm sure they're floating around somewhere out there.
However, if El Capitan does what you want it to do, you don't have to upgrade. you can if you want to, though. I liked Dark Mode so I upgraded to Mojave. No big deal.
However, with Catalina, there comes some teething pain. Let me explain. Mojave is the last OS to support 32 bit applications; in whole or in part. In Catalina, all applications must be 64 bit . If it's not, then it will not run. period. It must be upgraded to 64 bit in order to run, and that goes for everything. Catalina also excludes certain older models of Mac. Yours is not among them. If you wanted to, you could install and run Catalina fine.
Me, on the other hand, is a definite and official "no", seeing as how I have a Mac Pro (tower) from 2011. Does that bother me ?
not a bit.
so that's it....
John B