Mark has a 2012 iMac and a 2018 Mac Mini, along with a 2016 MacBook Pro and 12" iPad Pro. Can he hook up his Macbook Pro or his MacMini to his iMac and use the display? It can on a limited number of iMacs and is called Target Display Mode. IT requires a thunderbolt connection. Here's how - https://support.apple.com/guide/mac-help/use-an-imac-as-a-display-mh30822/mac. It may not work with Mark's model, but you can do it. Only for models mid 2011 to late 2014.