I just purchased a iMac 21.5" does it have Catalina or BigSur

All Depends on when Apple built the computer. I purchased a 2020 27" iMac right after the Apple Silicon Mac announcement. I need an intel chip to run windows virtually and needed a new mac as my 2015 MBP is aging. Mine came with Catalina on it. Fox is correct new macs would ship with the current OS but there is a short period of time when after the new OS announcement you could get the "old" OS.

I hope this helps.