stath

You can play music on a USB flash drive, SD card, or DVD, plugged into the car when you don't have a smart phone connected to the car. As soon as you connect your smart phone, all other media forms become invisible to the MB infotainment system.
I don't know if Mercedes designed this on purpose-- and why, or if it is a flaw of the system. But it was rather disappointing for this kind of car.
I have the base 2018 GLC with no MB Nav etc.