mini mac
Can a new mini mac be used with a 2015 Mac 21.5 " monitor or must you purchase a new monitor?
Can a new mini mac be used with a 2015 Mac 21.5 " monitor or must you purchase a new monitor?
You will need to get and use an external monitor (display) with the Mac Mini.
see > Connect a display to Mac mini - Apple Support
Your 2015 iMac does not support being used as a display in Target Display Mode,
plus there are no new Mac's including the Mac Mini can use a qualified iMac as a display.
please carefully read > Use your iMac as a display with target display mode - Apple Support
if it's actually a 21.5 inch monitor, it should work fine. but if you mean you have a 21.5 inch iMac, then, no, that can't be used as a monitor for the Mac mini.
mini mac