Wi-Fi connection between phone and car

Hello everyone, in anticipation of my ST Cupra, which is probably to be built in a few days, comes to mind again this question: How does the connection of the smartphone (I have an iPhone) via WLAN of the car work, what is it good for and….is it not that if the phone is connected per wlan, I can stream music about it (?), but then no more mobile data is sent/received, and so I am in a “wlan-sackgasse” (so said the Telekommann in the hotline, a Is I had WLAN, but the router “offline” was? Thank you for your answers and everyone a good slide!!!