How do Apple phones send messages to each other for free?

Both versions must be above 5.0. Open Settings-SMS, open imessage, both sides open it, and then open it in Settings-General-Network-Cellular Data, or open the search wifi network. The system will automatically scan each other. If the other party also has the network and imessage function, it will automatically send it through imessage, and the traffic will be very small.