Imac and Mac mini
Can I add a Mac mini to my Mac late 2013 model which is running Catalina and cannot be upgraded?
Earlier Mac models
Question marked as
No, you cannot use the iMac as a monitor for the new Mac mini.
The Target Display Mode feature has been deprecated by Apple a few years ago and is no longer supported in hardware or software.
There are third party solutions that might interest you. Search Luna Display and Duet.