When the phone is not connected to wifi and the cellular data is turned on, the cellular data traffic will be used. It depends on what functions are turned on in your phone. For example, if you turn on location, looking for your iphone, it's going to consume traffic.
You can click Settings - cellular mobile network, turn off the following unnecessary programs, then these programs can not use cellular mobile Internet. The specific traffic can be viewed by the system service in the cellular mobile network, which will have information about how much traffic each program consumes. It may not be the same as what the carrier calculated.