It doesn't. The API for displaying a notification is purely local.

Receiving a ping from Firebase Cloud Messaging triggers the app to whatever it does in order to display its notification. In the case of Signal, that probably means something like fetching the user's latest messages from the server, then deciding what to show in the notification based on the user's settings, metadata, and message content.

Here's example code for using FCM to show a notification. In this case, the notification content also passes through FCM, but Signal does not do that. https://www.geeksforgeeks.org/android/how-to-push-notificati...

Sorry I should clarify, by "it" I meant any sort of ping must go through Firebase Cloud Messaging, not that the message content itself goes through Firebase.

Looks like there is a way to bypass Firebase by using something like UnifiedPush which runs a perpetual background process that acts similar to Google Play Services to pick up notifications from the server and calls the local notification API.

It's theoretically possible to just keep an app running in the background all the time and periodically poll a server.

That's unreliable though since some OEM Android builds will kill it for that even if the user disables battery optimizations. Those OEMs sort of have a point; if lots of apps did that it would drain the battery fast.

Then that's basically what I said right? That there is in practice no way to opt out of using Firebase if you want consistent notifications.

Not clear what your point is. The Signal server wakes up the app via an empty message. At most the info this conveys is that a Signal app got a message to pull.

My point is there is no reasonable way to remove oneself from Google and Apple even with a fully custom application, they control the notification servers for their devices.

Yea sorry I didn't follow the thread properly.

When running Signal without google play services, Signal reliably received push notifications and with minimal battery drain.

Do you have microG? That provides a compatibility layer for FCM.

No, my intention was to not touch google services.

Any application can send notifications without going through a server.

How? I'm not talking about an application backend server but a notification server which Google and Apple have for all apps. I'm not sure besides polling or having a persistent connection to send notifications to an app while that app is not running.