Can I use my 2013 iMac desktop as a monitor for my 2020 Mac Pro?
Can I use my 2013 iMac desktop as a monitor for my 2020 Mac Pro?
iMac 21.5″, macOS 10.15
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Sorry, but as hcsitas has already mentioned, although the 2013 iMac would work, it won't work with any 2019 or newer Mac models.
Ref: Use your iMac as a display with target display mode - Apple Support