Phone. Apple Carplay basically just mirrors your iPhone.

 

It's horribly unintuitive, but in order to increase or decrease CarPlay's volume, you need to adjust it while CarPlay is speaking. Once it's done speaking, adjusting the volume only affects the car's volume, not CarPlay's.

I'm assuming this is what you're referring to – music volume is shared with the car's volume.

 

Oh I thought they meant what data is being used, the car’s LTE/5G or their phone’s 😅