Connecting Firewalla Purple to a Wifi Enabled Vehicle

I have been trying to find out more about the security of car wifi, Apple CarPlay, and Apple Share Play, especially  as typically they are all going to be used simultaneously.

I ask this as I have some very good friends wit not much of a radar for network security,. While I trust them I do not trust where they have taken their phones and devices.

I understand that Car play functions similarly to AirPlay, where data is just mirrored on another screen. Presumably  this uses the car's WiFI radio to achieve this? I could not confirm this assumption on my own.

Enter SharePlay while in the car. Are people connecting to  this service over the car's WiFi as a LAN, are they connecting to (in this case) my iPhone, or are they connecting via their own cellular radio to a tower, then back to my iPhone?   The answer might be yes to all and SharePlay will use whatever is available.

Based on its function, I would guess the car Wifi has some kind of router functioning, and I do not want my friends phones on my car network without knowing more about its security features: SO, how would I connect a Purple to the car Wifi LAN, and then what settings might need to be adjusted on the FwP to allow users to connect to car WIFI for SharePlay through the FwP?