Setup another display
how do I connect another display to my I Mac when the display has a cdmi cable
Question marked as
What year model iMac? (the iMac display output ports have changed 3 time in less than 10 years)
Identify your iMac model - Apple Support
If it is a new/newer iMac, then Apple sells two different USB-C to HDMI adapters.
the > USB-C Digital AV Multiport Adapter - Apple
and the > Belkin USB-C to HDMI Adapter - Apple
for more info, see > Connect an external display to your iMac - Apple Support