My iMac is a 2019 model fitted with the 3.6GHz 8-core i9 processor. I think it's the age of the older Mac that is significant. Mine is a mid-2011 model and works like a dream.I thought I read you can use an old Mac as a second screen but your primary mac needed to be suitably old as well?
So in the picture I posted, are my sockets with the lightning bolt icon, lightning or thunderbolt connections? Same icon for both I believe?
It's a 2017 machine I think. USB C looks similar but I don't think its that.