With a small camera to scan qr codes from others? That part is actually simple. And the connection direct wifi.

People could also scan bill amounts or type numbers in. I see it easier thinking hardware wallet on a wrist than a full android device, but that could be done too.

Of course the point is that it could work without any other device around, otherwise it makes no sense. Its fine to do some minor syncing and configuration at home but that should be it.