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You do it with Target Display Mode. You're right on the cusp of compatibility here! TDM only works on <2014 iMacs (so it will work on yours). According to Apple documentation, these are the requirements:

  • The iMac used as a display must have macOS High Sierra or earlier installed.
  • The other Mac that you're connecting it to must have been introduced in 2019 or earlier and have macOS Catalina or earlier installed.
  • The cable connecting the two Mac computers must be a Thunderbolt or Thunderbolt 2 cable.
(Source: https://support.apple.com/en-us/HT204592)

Basically you connect a Thunderbolt 2 cable from the Mac mini to the iMac, and then press Cmd + F2.

Hope this helps.
 
I have TDM working on a 2013 iMac using Monterey and OCLP. The other computer is a Mac mini 2018.
 
Only if the iMac does not have Thunderbolt.


If the iMac has Thunderbolt, then the iMac can only be used as a display for other Macs and you need to use a Thunderbolt 2 cable to make a Thunderbolt connection.
If the Mac you want to connect to the iMac has Thunderbolt 3 or Thunderbolt 4, then you need to connect the Thunderbolt 2 cable with an Apple Thunderbolt 3 to Thunderbolt 2 Adapter.
 
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What cable should i need for a iMac Mid 2011 with High Sierra either with Mac Mini 2018 or Mac Mini M1?
 
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