Every few minutes, Briar tries to connect to your contacts via Tor, wifi and Bluetooth. If multiple connections are made, Briar keeps one connection to each contact via each transport and closes the others (with one exception: if Briar connects to the same contact via Bluetooth and wifi, the Bluetooth connection is closed).
Messages are exchanged with each contact over whatever connections are available. If multiple connections are open (for example via Tor and wifi), they're used in parallel.
When adding a contact in person, all Bluetooth connections to other contacts are temporarily closed to ensure that a connection can be made to the new contact.
If a contact is outside wifi or Bluetooth range, Briar doesn't use other devices that are in range to relay data to the contact.