How do I connect my Apple TV to my iMac so I can watch whatever is on my Apple TV

You can buy a video capture card that plugs into the USB port on your iMac you would then use the HDMI cord to plug-in your Apple TV to the video capture card device in which your iMac would then mirror it’s what I used to play my PS five on my 20 13 27 inch iMac.

Here’s a link to one you could use

https://a.co/d/byDWCT8

You can't use your iMac as screen for the Apple TV box. Connect the Apple TV box to a dedicated screen (TV/​monitor/​projector). iMac doesn’t take input from HDMI devices.

You can use on your iMac most services that are also on the Apple TV box, through equivalent apps or their web access counterparts.

Video capture is blocked for commercial content providers (movies and TV shows), through HDCP. Video capture wouldn’t work for streaming apps.

Gaming might be different.