The Signal server would send a backend notification to the client app via the Apple Push Notification Service. The app is then able to wake up, at which point it fetches new messages (securely) from the Signal servers. The app then generates a local notification with a preview of the received message. iOS is then logging those messages.

Oops. I basically duplicated your reply. You are spot on the mark.

So long Facebook, see you never, until one day I inexplicably need to use your platform to get updates from my kid’s school.

Screw any school or PTA that distributes information by Facebook. That should be outlawed.

What, don’t you still use it to unpack warez?

I think Raspberry Pi is in the class of SBC (Single Board Computers). This is a PC, apparently. Apples and oranges.