One hearing aid picks up a sound and sends to the other one "Did you get that sound?" The other one sends "No, I haven't got it yet, must be on your side" and adjusts its volume accordingly, all in the time the sound takes to travel the width of your head.
Modern hearing aids are invisible unless you're peering into someone's ear, which is rude at the best of times. For some reason, old people don't want to have hearing aids for vanity reasons, "it makes them look old", but they will go for years with their hand cupped behind their ear saying "Eh? What's that you say? Speak up, sonny!" and think THAT doesn't make them look old .... !
I'll get 'em when I need 'em ...