Briar routes all internet-based communication through the Tor network by default - there is no option to use clearnet. When two Briar users communicate over the internet, each device runs a Tor hidden service, and messages are exchanged directly between these hidden services. No central server ever sees your messages, metadata, or contact list.
This architecture means there is no single point of failure or surveillance. Even if an adversary compromises a server, there is no server to compromise. Each device is both a client and a server, communicating through Tor's onion routing. The Tor hidden service model also means neither device needs to reveal its IP address to the other.
Briar can also communicate over local Wi-Fi and Bluetooth when internet access is unavailable. This makes it uniquely valuable during internet shutdowns, natural disasters, or in areas with unreliable connectivity. Messages sync automatically when a connection is re-established.