Can mac mini be connected to your old iMac?
Can mac mini be connected to your old iMac?
iMac 21.5″, macOS 10.15
Question marked as
If you're asking about using the iMac as a display, review the document: Use your iMac as a display with target display mode - Apple Support
Target display mode is a feature available with late 2009-Mid 2014 iMacs. The display Mac has to be running macOS High Sierra or older and the signal producing Mac has to be running macOS Catalina (ca. 2019) or earlier.
If you're asking about file sharing, there's various ways depending upon what kind of sharing you need to do, such as one-time or constant.
Transfer files between two Mac computers using target disk mode - Apple Support