Is it possible to use the 950XL as a media device, meaning that when the USB port is plugged in the phone appears as a media device (hard drive with media) on a car infotainment system (like a iPhone does)?
Yes, Most Lumia phones Media content, either stored on the phone Mem or sd card will be available to play on a third party device.
The 950 uses C-type USB connection, if you have a C-type to USB, any will do as USB (3.1, 3.0, 2.0) is backward compatible.
Should have said C-type USB is also compatible to USB 3.1, 3.0, 2.0 and 1.0