What I’m looking for ultimately is a universal chat type app like Beeper that can handle Signal and SMS, however, reading this about it gives me pause. It would be nice if I could get all my peeps on matrix, but since it was so hard to get them on to Signal, I think the best I can hope for is something than can handle matrix, signal, and sms. Which brings me back to the title, how exactly do Matrix bridges work and are they secure?
EDIT: SMS is insecure by its very nature, yes?
I just want to clarify that most bridges can be set up to have E2EE between the Matrix client and the bridge (regardless of whether the bridge supports encrypted chats on the bridged service because not all do, e.g. Facebook), but it is true that the bridge itself has to decrypt and translate between Matrix and the 3rd party chat service, so as you mentioned trusting who hosts bridges or doing it yourself is really important.