macOS has three features that sound close to Second Display. Each does something different.
AirPlay
With AirPlay Receiver turned on, a Mac with macOS Monterey or later shows what another Mac, iPhone or iPad sends it over your network. An iMac can receive AirPlay from the 2019 models on, and so can the iMac Pro. Apple: Continuity requirements
Sidecar
Sidecar makes an iPad a display for your Mac. It doesn’t work with a Mac as the display. Apple: Use an iPad as a second display
Universal Control
Universal Control lets the keyboard and mouse or trackpad of one Mac work on up to two other Macs or iPads nearby. Each keeps its own screen and apps, so the iMac doesn’t become a display for your Mac. Apple: Use Universal Control
When Second Display fits
- You want the iMac as an extra display of the Mac you work on, with your windows on it.
- You want a lossless picture over a Thunderbolt cable. We measured 60 frames per second on an iMac 24‑inch with M1; older and Intel iMacs aren’t measured yet.
- Your iMac is from late 2015 or 2017: too old to receive AirPlay, new enough for Second Display. Which iMacs can be a display