2015 21.5 iMac as monitor for Xbox One?

Usually one could only use an iMac as a display with another Mac and use a screen sharing utility.

Or to administrate headless remote servers, there are other utilities that allow that aspect. A usual

answer would be to get an inexpensive display, instead of running an expensive iMac as one.

Good luck in this matter! 🙂

The 2015 models do not have the ability to act as a stand alone monitor. Use your iMac as a display with Target Display Mode - Apple Support