how to use old iMac as monitor for a mini mac
How to use old iMac as monitor for a mini mac
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Only a small sub-set of iMacs worked as external monitors for other Macs. See if yours is included in the supported models in this Apple article:
Use your iMac as a display with target display mode - Apple Support
Also note in the article that 2020 Macs can not take advantage of any iMac as an external monitor.
The article does not include the requirement that the iMAc is working normally. Dead/broken ones need not apply.