How Bluetooth communication between devices works
Bluetooth is a short-range radio. Here is what that means for local messaging and for offChat.
Direct answer
Bluetooth is a short-range radio on the phone itself. Two phones that can see each other may exchange data with no internet and no router. Typical range is metres to tens of metres, not kilometres.
What Bluetooth is not
It is not a worldwide network. It does not punch through every obstacle. It does not replace a carrier. Interference, a mis-set airplane mode and denied permissions all break discovery.
In offChat, Bluetooth is the short layer: nearby phones that already meet on the radio.