The tagline for Luna is "Use any Mac as a
second display for your Mac or PC". In fact the
very video I linked to has (description: Have you ever wanted to use your old Intel iMac as a second display or main monitor for your M1 Mac Mini? Well, Today is your lucky day Thanks to
Luna Display from Astropad, you can turn your intel iMac into an M1 Mac Mini. This very cool device allows you to use your iMac as the Main Monitor for your M1 Mac Mini or even as a second display for your Mac Mini M1 desk setup.)this in the comments:
Q: How would this work
if all you had was the old iMac and the M1 MacMini? Also are drives connected to the iMac accessible to the M1 or do they have to be through the M1?
A: For the
initial setup you need to connect the M1 Mac Mini to a monitor or TV and install the app and enable the settings. Drives need to be connected to the M1 Mac Mini.
As I said "the one issue with that idea is you can't use an old iMac as a monitor for the Mx Macs
out of the box."
Never mind you need $120 piece of hardware to do this.