if cellular data is on and wifi is available which would be used by your phone to access the internet?
iPhone 6, iOS 9.1
When connected to WiFi, WiFi will take precedence when the phone is not in standby. If the phone is in standby and not connected to power, cellular data will be used. If WiFi assist is enabled (which it is by default), cellular data may be used when connected to WiFi if there is a problem connecting via the WiFi network.
Generally Wifi is used over Cellular when available and has a good signal.
If the Wifi signal is very poor, then Cellular may be used instead if Wifi assist is turned on.
About Wi-Fi Assist - Apple Support
Note that good signal is not the same thing as a good internet connection.
You can have very good wifi signal and still have very poor internet connection. Wifi Assist only works if the actual wifi signal is poor. Internet connection status is not considered.
cellular data usage
