spammie27 wrote:
My 2023 iMac shows up to extend my screen on my 2015 iMac, but not the other way around, which is what I want. Is there a way to make it show up and work?
The others who have responded so far have told you why Target Display Mode won't work.
From your description, I think what you're talking about is AirPlay. The reason that you can AirPlay between these Macs only in one direction is that AirPlay to Mac is a much more recent feature than AirPlay to an Apple TV set-top box.
Both of your iMacs know how to AirPlay to an Apple TV set-top box. The 2015 one might only be using AirPlay v1, but that would probably be enough to make some connection (albeit a lower-quality one) to a 2023 iMac acting as an AirPlay Receiver. Only the 2023 iMac is recent enough to act as an AirPlay Receiver. There may be third-party software that you could install on the 2015 iMac that would turn it into an "AirPlay Receiver" of a sort – so that you could use it as a second, low-quality screen for the 2023 iMac. My guess is that most such software will be using the older AirPlay V1 protocol that is limited to 1080p resolution.
Continuity features and requirements on Apple devices - Apple Support
Set up your Mac to be an AirPlay Receiver - Apple Support
Note that even between two Macs that are new enough to support AirPlay V2 and AirPlay to Mac, AirPlay is not as good as a real hardware monitor connection.
Is there even a way to use my old iMac as a second monitor or am I trying to do something that won't work?
There are third-party workaround products – Luna Display (Astropad) and Duet Display. Whether the costs and drawbacks of those are worthwhile in your case is something that you would have to evaluate.
