When using Mobile AppMeasurement libraries, a Carrier report is created. There is also a Mobile Carrier report under Technology > Visitor Profile.
The Carrier report from the native app library pulls the carrier name the device is associated with. It pulls this info regardless if the device is using that carrier then or on WiFi. All traffic, WiFi or not is sent in as that carrier. For this reporting type, you have high accuracy on associated/contracted carriers for that user/device. This report does not distinguish WiFi or carrier data at the time of the session. This report is also only available for traffic originating from an application using AppMeasurement libraries.
The Mobile Carrier report is calculated by looking at a unique key derived from a combination of domain and ISP. Using this method, breakdowns of mobile traffic by known carriers and connection type are available to see WiFi by carrier traffic. The drawback from this method is that many carriers mask their mobile domains via mobile proxy.