How exactly do apps not running "in the background" receive notifications?

Question

So, Android has this view (I think left to the home button is standard) where you see all the "currently open" apps, whatever that means exactly. What I'm wondering is, how can apps that are not amongst the currently "running" ones receive notifications? For example, Whatsapp, Reddit or Email will still send alerts even on a freshly restarted smartphone where I definitely didn't start the app manually.


How exactly does this work? Are, in a sense, all installed apps running in the background all the time, and in regular intervals asking their home server if there has been a notification? Does the notification somehow get sent to the Android system itself, which in turn activates the requested app on demand? At least on a PC I would expect that a program has to be running in order to send alerts, but maybe Android is completely different?


— MaxD | Sources

Answer

Notifications of Whatsapp and Reddit are triggered by push notifications sent from Google servers, received by Google Play Services, a system app which always runs in background. All Google Push notifications (or as they are named today "Firebase Cloud Messaging" notification) are received and initially processed by the Google Play Services.


The next processing step of the received push notification depends on the push notification type. Plain text notifications can be directly displayed without even starting the app it is targeting.
Other push notifications are delivered to the app it belongs to (which is automatically started if it is not already running) and the app can then "translate" the push message into a notification (in case of Whatsapp this require e.g. decryption of the received message or even fetching additional data from Whatsapp servers).


— Robert | Sources

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