how do I know if my Mac mini is 32 bit or 64 bit?

If you click the Apple icon in the top left corner, and select About This Mac, it shows you what type of processor the Mac has. The only 32-bit processors that Apple has used in Macs are Intel Core Solos and Intel Core Duos (to the extent of my knowledge), so if you have an i3 or i5 or i7 or something like that that is not one of the two mentioned above, I'm pretty sure its 64-bit.

Macs have used 64bit CPUs since 2005. macOS is 64bit only since Catalina.

You're welcome.