I purchased this reconditioned 2018 Mac mini to use it with my 21.5 inch mid 2010 iMac as a display it is a very nice machine.  I did considerable research online and through Apple to make sure they were compatible.  I purchased the correct cable with a mini display port end and a thunderbolt USB – C end.  I followed the instructions in the videos exactly but nothing happened. I tried this and other things they suggested, but still, I was in the dark unfortunately not everything is meant to be. I am very sad. I was hoping it would work out.. As of right now, I will have to return the Mac mini and we will go our separate ways.-