On-device bridging works like this https://blog.beeper.com/p/how-beeper-mini-works. We'll put together a full technical deep dive for the real launch, this is just an open beta. Our signal bridge code is open source: https://github.com/mautrix/signal

You don't have to use our hosted bridges, we've made it ridiculously easy to self host: https://github.com/beeper/bridge-manager