How Carplay actually works

Thanks for reply - that’s great info about the usefulness of Carplay - when it works. My query was really about how it works via bluetooth, Wifi and hotspot. It seems that a bluetooth connection must initially be established between the phone and the head unit. Car play must then somehow run via Wifi as the bluetooth connection between the phone and the head unit switches off once Carplay launches on the Android Head Unit screen. But, when in my driveway, the phone itself connects to the internet via an automatic wi-fi connection to my home modem. So, with Bluetooth gone off, how is the Head unit simultaneously connecting to Apple Music for example?

When I drive away from home, my phone no longer connects to the internet via my home modem and defaults to my mobile phone service provider’s internet connection. So how then does Wi-fi apply? Is this a separate Wi-fi connection whereby the phone operates as a sort of modem for the Head Unit to connect to the internet?

And where does “Hotspot” come into the picture. Is the Head Unit connecting to the phone via the phone’s Hotspot? So must this be turned on too?

And why might my screen be showing Apple Music but be playing music from my original car radio or CD player instead?

Thanks to anyone who can explain.