2020 iMac 27 in used as external monitor

No the 2020 iMac does not support Target Display Mode.

Use your iMac as a display with target display mode - Apple Support

AirPlay might be an option for using the 2020 iMac as a second display, but you will still need a Studio Display or aftermarket external Display as the Primary Display.

AirPlay - Apple

Nope, the 2020 27" iMac is too new. If you are planning on getting a Mac Studio you need to buy an external display, there are many to select from.

You’re welcome.