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?
…as long as you keep them up to date and follow some basic security practices. There is nothing stopping you from self-hosting an outdated vulnerable version exposed to the public.
Third parties are a risk of unauthorized access, but may be more likely to follow security practices in order to avoid getting fined (according to the legislation of wherever they’re hosted).