How is audio transmitted with wireless Carplay?
Apple’s consumer documentation says you need a unit with Bluetooth or WiFi for wireless and I expect it uses both when paired and available. You’d need to measure the RF to tell which is being used from moment to moment or check console logs for any debug messages. 


https://support.apple.com/en-us/HT205634


A slightly longer answer is that the technical details are not public and neither is the API / SDK other than this technical detail that explains the app simply hands off the media container or media with title, streaming Boolean, artwork and percentage played...


https://developer.apple.com/documentation/mediaplayer/mpcontentitem


The details on the SDK are shared with MFI partners. 


https://stackoverflow.com/questions/22372095/is-ios-carplay-api-public-how-to-integrate-carplay/38541642#38541642


As a developer, you don’t need to be MFI, just hand off your media to the player and it will play through. As a user, you can control what media you ask to play but probably not affect the delivery process in any meaningful way other than choosing your source media to be of a quality you expect to hear.