On Android the OS implementation of "push" notifications is pull/poll based as well. At some interval, the OS polls Google's servers to see if there are any messages available. Firebase essential acts as a message broker, so that it only has to poll a single server, instead of a separate server for every service that wants to send notifications, and there is only a single service polling.

But I really wish Android supported specifying additional servers to poll (and/or replace the default server), so you could use a self-hosted service in addition to or instead of Google's service.