Can I use my iMac as a monitor for my Xbox?

You cannot use your iMac 21.5” as a monitor for an Xbox, Windows computer, Linux, macOS, OpenBSD, nor any other system.

You cannot use your iMac 21.5” as an external monitor at all.

Your iMac 21.5” is not a monitor, and cannot pretend to be one.

The only iMac models that can provide were specific older 27” iMac models that pre-date the Retina graphics support.

If you wish this feature to become possible with future iMac models, log some feedback with the folks that work at Apple:

https://www.apple.com/feedback/

No and no work arounds.

Let's just simplify the answer.

No.

It will not work, not with a cable, not with magic and not with rope. IT WILL NOT WORK! Your iMac is a computer, not an external display for an Xbox.