Using a Mac as an External Display for Another Mac

This is a first pass at trying to come up with a general solution to using a Mac as an external display for another Mac. The idea is to reuse old equipment or supporting more monitors than a Mac directly supports. My tests show that older Macs can be used as displays for Macs running Big Sur or Monterey. I have further testing to see if it works on older operating systems like Catalina, Mojave and High Sierra. You can do this using AirPlay if your systems are running Monterey but many would like to use older, and sometimes much older systems as displays for newer systems. Sorry for the video quality - this is the first time I've done a video using a camera in a couple of years and it took me a while to find a usable mount. The video overall took about 7 hours as I had to install El Capitan on the old MacBook Pro and do multiple takes. At least I learned a lot about doing videos with cameras. Link to a detailed video describing how to set up a virtual display:    • Using a Mac as a Display For Another Mac