iMac as only monitor for MacMini - the process downsizing with externer SSD

Thanks for the comments.

I have misread the info about this on the internet. The iMac could work as monitor but my MacMini is too new :-(

I think the best solution is to buy a good separate new monitor.

:-)

I have a iMac late 2012 (21,5 ") I bought a used MacMini 2022 and want to use the iMac as a sole monitor för the Mini. I have the thunderbolt ready. I know I should downsize my Catalina on the iMac to HighSierra OS for the "target display mode" to work.

I also have the main OS on my iMac on an extern SSD and can choose between the "main, old iMac" or the new SSD when I start up the system. Both setups of course run the same Catalina. I didn't set it up got help for it.

I think I know how to reset the iMac Catalina to HighSierra OS.

But how do I choose so the reset affects only the iMacs harddrive and OS and not the SSD? And how do I( if I get the iMac to function as monitor for MacMini) then transfer the main setup (programs, data) with Catalina from the SSd to the MacMini?

Edit: I opened the thunderbolt cabel box and it isn't the right one for the iMac?? Is it the "thunderbolt 0" on my iMac and the thunderbolt 3 on my Mini? I now have a thunderbolt 3 - what do I need to get the right connection done to the Mini? It gets more and more complicated...

Thanks and greetings from Finland!

Berndt Holmstrom

Edit: I opened the thunderbolt cabel box and it isn't the right one for the iMac?? Is it the "thunderbolt 0" on my iMac and the thunderbolt 3 on my Mini? I now have a thunderbolt 3 - what do I need to get the right connection done to the Mini? It gets more and more complicated...