If it’s anything like Beeper 's Matrix bridge then it’s E2EE Matrix encrypted between your device and the bridge server and then using Apple’s iMessage encryption between the bridge server and Apple/the other user.
The weak point is always going to be the bridge software as by necessity the message must be decrypted there to re-encrypt for iMessage.
At least in Beeper/Matrix the bridge software is open source and one can host their own bridge while continuing to use the existing Beeper/Matrix main server.
Doing so gives you no-trust security since the Beeper/Matrix host cannot decrypt the messages between you and the bridge you control and rubbing your own bridge eliminates that weak point.
If it’s anything like Beeper 's Matrix bridge then it’s E2EE Matrix encrypted between your device and the bridge server and then using Apple’s iMessage encryption between the bridge server and Apple/the other user.
The weak point is always going to be the bridge software as by necessity the message must be decrypted there to re-encrypt for iMessage.
At least in Beeper/Matrix the bridge software is open source and one can host their own bridge while continuing to use the existing Beeper/Matrix main server.
Doing so gives you no-trust security since the Beeper/Matrix host cannot decrypt the messages between you and the bridge you control and rubbing your own bridge eliminates that weak point.