There are hardly any alternatives to the Apple Studio Display that combine a 5K resolution with a built-in webcam. However, Luke Miani has repurposed an old 27-inch iMac as an external monitor, creating a cheaper version of the Studio Display in the process.
https://www.notebookcheck.net/Apple-s-27-inch-iMac-converted-into-a-cheaper-Apple-Studio-Display-with-a-5K-resolution-and-a-built-in-webcam.616430.0.html