can I use my 2021 iMac as a monitor for my xbox?

What you want—iMac Target Display Mode—is dead tech. See the very restrictive hardware and operating system requirements for Target Display Mode here:

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

Apple started its deprecation nine years ago and gave it the coup de grâce in Early 2020.

Even when TDM was a thing, it was never intended, documented, not supported for non-Apple devices.

No sorry you can not. The iMac is a computer and not a display for other devices including other Mac's.