I would like to use my old iMac 2012 as a second monitor for my Mac mini
I would like to use my old iMac 2012 as a second monitor for my Mac mini
iMac 21.5″, macOS 10.15
If you have a new M1 Mac Mini, you cannot. Your Mini must have been manufactured 2019 or earlier and must be running macOS Catalina or earlier. Modern Macs no longer support Target Display Mode. See the link below for the rules of engagement.
Use your iMac as a display with target display mode