I don’t have a direct source other than the source code of the software they use: https://github.com/mautrix/signal

When using one of their “cloud hosted” bridges, the bridge software (that connects between Matrix/Beeper and other protocols) has to read all message content. Otherwise, it’s impossible to bridge to another protocol. E2EE becomes end (other users) to bridge (beeper) encryption.

With “local hosted” bridges, E2EE stays intact, but messages can’t be sent/received if the device hosting the bridge is unavailable.

In the future, with MLS (a different E2EE protocol), it could be possible to keep E2EE even when bridging to Matrix on cloud hosted bridges.

The linux binary is being used.

Try Proton. Currently, Proton is much more developed than the Linux support for many game engines.