Question about Digitec USB-C - HDMI adapter

This is not due to the TV, but to the mobile. Not all mobile devices support the output of screen data via USB. It is crucial that the device's USB-C port supports video output. This adapter only works with devices that offer DisplayPort Alt mode. You can find out whether your device supports DP Alt mode in the technical data for your mobile. Or simply ask Google about it...

Thanks for the answer.
Why does the mobile phone have to support DisplayPort Alt mode if the source is HDMI?
It works with this adapter from Apple and it is only USB-C to HDMI.
Apple Multiport (USB-C, HDMI, USB-A, USB-C)