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?