This document focuses on how a pen display connects to a mobile device such as a phone. It's a subset of the broader guide Connecting a pen display to a computer, but focused on the specific issues that come up for a mobile devices.
USB-C port on the mobile device
Your mobile device must have a USB-C port
Your mobile device's USB-C port must support DP alt mode. Not all USB-C ports support DP alt mode.
USB-C cable connecting the phone to the tablet
This cable must also support DP alt mode
Compatibility between pen display and a phone
Even if your phone's USB-C port supports DP alt mode, your phone simply may not support using it with a pen display. A great example here is iPhones - they simply do not work with pen displays at all.
A pen display may be compatible with some mobile devices but not others. You should contact the manufacturer and ensure which phones they support with their tablets.
Power
Your pen display needs power from somewhere.
Most likely your mobile device cannot provide enough power so you will need get power separately.
Apple iPhones - these phones DO NOT WORK with pen display