How can I upgrade my iMac to macOS 11 or 12?

What does it say in the About This Mac dialog in the Apple () menu? Model name and hardware year/sub-year – no serial numbers, please.

If the machine is a 27" iMac with a model year of 2013 or earlier, it can't run Big Sur or later. If it has a model year of 2014 or later, that model year will tell us how far it can go.

This won't help with "Banana Plus", but you may want to consider downloading the current version of the Firefox Web browser. It is still receiving updates, unlike Catalina's version of Safari.

https://www.mozilla.org/en-US/firefox/new/

It is the year model that determines the highest available macOS.

What year model is your iMac?

see > Identify your iMac model - Apple Support

FWIW, you need one of these Macs to upgrade to Big Sur or later…

macOS Big Sur is compatible with these computers - Apple Support

It is iMac late 2013,

You are maxed out with Catalina