Use iMac 27” as external monitor
Can I use my 2012 iMac 27” as an external monitor for my 2020 iMac
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NO.
Only Macs made up to 2019 can connect to another
iMac in Target Display Mode.
"You can connect any of these older iMac models to another Mac from 2019 or earlier for use as an external display. Learn how to identify your iMac model."
From this document:
Use your iMac as a display with target display mode - Apple Support