upgrade from macOS Catalina

sorry, no. Catalina is as far as that iMac can go. you need a 2014 and later iMac to upgrade to Big Sur.

two things to consider:

there will likely be one more Catalina update before it possibly becomes "unsupported" after Ventura is released.

and

if it still works for what you use it for, you can keep using it. when I bought my current iMac, I gifted my old one (late 09 model) to my dad. it cannot upgrade beyond high Sierra, but it still does everything that he needs from it. and he loves it.