Use an iMac as an extended display ...

rlesperance wrote:

With Sequoia, it is possible to extend a display using other Apple devices. Can I do that with a Mac Mini and an iMac Intel 27" ?

Assuming that you are talking about a new or recent Mac mini, not with Target Display Mode. Target Display Mode went away for good in Late 2014 when the first 27" 5K Retina iMac came out. Even if you have a 27" iMac released between Late 2009 and Late 2013, Apple added restrictions on newer Macs and versions of macOS such that none of them support using ANY iMac as a Target Display.

See the bad news here: Use your iMac as a display with target display mode - Apple Support

If that is a 27" 2019 or 2020 iMac, you could set it up as an AirPlay Receiver for AirPlay to Mac. Note that AirPlay to Mac is not nearly as good as a hardware video connection. I don't think it will let you use the full 5K resolution of a 27" 2019 or 2020 iMac, and there might be lags and artifacting.

Continuity features and requirements for Apple devices - Apple Support

Note the very restrictive conditions stated in the Target Display Mode article that rkaufmann87 links. TDM is no-go, as its specs only fit older hardware and macOS versions on both the display end and the video source end.

NO third-party workaround for using an iMac as a Mini monitor will work. All, including Airplay to Mac, are laggy. That might be ok if the iMac is a secondary monitor used only for parking static windows and app palettes, but NOT for the primary display

If you want no lag and reasonable video options, the Mini needs a standard, free-standing external monitor. When I tested TDM with older Macs that were both within the the limited scope of the the TDM sysreqs, I found I could not extend the desktop, only mirror it.

Target Display Mode has be DEAD for over 10 years now. Please remember that iMacs are computers, they ARE NOT external displays for other devices. If your MM needs an external display, please buy one. You can easily find external displays beginning at <$100 (try Costco if you are in the USA), however for a nice quality display expect about $300.

Target Display Mode

Yes: Extend or mirror your Mac desktop across multiple displays - Apple Support

To be clear, are you contemplating buying a Mini and using the iMac as its external display? That's not going to work. It certainly won't work for initial setup for example. For that matter I am less 100% certain it will work with an Intel Mac. I am 100% certain the Extend feature will work with M series Macs.

It's not a requirement for both to be running the same OS. They will both need AirPlay Mirroring but that's been a macOS feature for quite some time.

The Apple Support specialist should have referenced the following excerpt (emphasis added):

"If you’re using a device with no built-in display, such as a Mac mini or Mac Studio, you must have more than one display connected to see these options."

That was the reason for my followup question. Apple Support documents generally are written with great care and they are intended to be interpreted literally.

If an Apple Support document does not explicitly state something is possible, it has been my experience that proposed "something" is not guaranteed to work, and is likely to be impossible. That doesn't stop people from assuming they can do things that can't be done. In the rare event they're successful they gleefully post YouTube videos and perpetuate the myth. Then Apple comes out with a software update, spoiling the fun.