Can I use my late 2015 27" retina 5k iMac as a monitor for a 2024 iMac mini?

The very restrictive hardware and macOS requirements for the now-deprecated Target Display Mode—what you are asking about—are laid out in this Apple article:

Use your iMac as a display with target display mode - Apple Support

Both your devices are far too new. Apple started the deprecation of TDM ten years ago and threw the last shovel of dirt in its face in early 2020.

No. If you get a new Mac mini, then you need an aftermarket external display.

see > Connect a display to Mac mini - Apple Support

Servant of Cats wrote:

There is a third-party workaround product called Luna Display (by Astropad) that might let you reuse that 5K screen as a second display, but the workaround is not as good as a connection to a real hardware monitor. I think the Luna Display people themselves say that the product is best suited for connecting a second reference screen where you are not going to be displaying rapidly-changing video content (e.g., for editing videos, or playing games).

Even if you were interested in that, you'd still want/need a real hardware monitor for your first monitor.

Re Luna: you are absolutely correct. This is the advisory on Luna's support page:

That says to me it is unsuitable as a primary display. If you need space for floating palettes, open folders, and the like, it may be useful. Otherwise, no.

Neither of your Macs support Target Display Mode.

Target Display Mode went away in Late 2014 when the first 27" 5K Retina iMac came out – and never came back. In addition, Apple added restrictions on "the other Mac" so that Macs released after 2019, or that are running versions of macOS newer than Catalina, do not support using ANY iMac as a Target Display.

Your Late 2015 iMac is also too old to act as an AirPlay Receiver for AirPlay to Mac.

There is a third-party workaround product called Luna Display (by Astropad) that might let you reuse that 5K screen as a second display, but the workaround is not as good as a connection to a real hardware monitor. I think the Luna Display people themselves say that the product is best suited for connecting a second reference screen where you are not going to be displaying rapidly-changing video content (e.g., for editing videos, or playing games).

Even if you were interested in that, you'd still want/need a real hardware monitor for your first monitor.

Not possible unfortunately, the iMac is a computer, not a display. Use a regular monitor for this.

Mac mini it’s a desktop computer as iMac too you can’t use a iMac for Mac mini as external monitor , only MacBooks