> I'm not up to date with Android, but why does displaying notifications require an entire server or a paid service? Is it not just a matter of installing a client program and sending a curl to an endpoint?

Imagine if every program on the users phone had their own schedule for checking for updates against remote hosts. That sounds terribly inefficient and wasteful of power, as the applications would need to "wake up" (as in, consume processor cycles) to initiate and perform a check.