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?
There are many different types of bridges, but the most seamless one is a type of Man In The Middle (MITM). You give the bridge full access to your other services, which allows them to copy everything to Matrix and vice versa. Naturally, this circumvents E2EE as the bridge needs to access and manipulate the content somehow (E2EE only exists up to the bridge, not the whole way to your client). The bridge can theoretically do anything, as it is a MITM. However, because most bridges are open source and you can host them yourself, the risk that unauthorised parties can gain access to the data is fairly low. If it’s hosted by a third party, you have to trust them that they won’t abuse their power.