sonicadvisor wrote:
So then I'm gathering my iMac (Retina 4K, 21.5-inch, 2019) is not meeting the system requirements then :(
Many thanks for your help Servant of Cats.
That iMac is new enough ("iMac introduced in 2019 or later") to meet the requirements.
That isn't the only requirement. For instance, you need to be running Monterey or later on it before you can set it up as an AirPlay Receiver. You need to turn on the AirPlay Receiver feature in the iMac's System Settings before trying to make the screen mirroring / AirPlay connection from the MacBook Air. Etc.
You didn't say which model of MacBook Air you have. It would need to be one "introduced in 2018 or later" for "best results" (or for it to act as the AirPlay Receiver). But even an older MacBook Air might be able to AirPlay to that iMac with "a lower video resolution."