Upgrade of late 2015 imac

I have an iMac (Late 2015) with 1,6 GHz Intel Core i5. (Memory 8GB) It is running on Sierra 10.12.6 (NOT High Sierra). I would like to upgrade but am confused. Apparently Monterey is available for my iMac if it is a "Retina" version - is this correct/does it matter/is it the same? I would obviously like to upgrade to the newest system possible. Maybe Monterey is a bad choice? Can anybody help? I am a pretty basic user .......