Every doorbell will have a Bell Code which is a UUID rendered as an easy scannable QR code. When the app first starts it will connect to Firebase in the background, to obtain a messaging token (a unique code that identifies that installation). When scanning a Bell Code the app then registers that UUID and token at the openbell.org API.

When a doorbell is to be activated, the doorbell (or you manually) make a request to the openbell.org API with the UUID of the bell being pressed. The API will look in the database to find all registrations for that UUID, retrieve their tokens, and send those to Firebase to send the message to the apps which have registered. This allows a single doorbell to be registered to send notifications to multiple apps. An app can also be registered for multiple doorbells.

The list below will manually inform the openbell.org API that the UUID has been activated and begin the process of sending messages to all the tokens registered who have subscribed to the relevant doorbell. If you wish to create a physical doorbell to do this you should study the simple HTTP request and know how to send it to the openbell.org RESTful API. There is no central registry of Bell Codes; doorbell manufacturers are free to generate any valid UUID they wish.