Push notifications work through Google/Apple services?

Question

Android

It is necessary to set up Push notifications for the mattermost client on Android: there are already a lot of incomprehensibility: why do you need some kind of "notification proxy" for push notifications, why can't you send this notification from inside the application? In general, we are invited to use their proxy, or set up our own.
Naturally, for security, this is not even discussed, and we choose the second option.
Okay, let's set up a proxy.
And then the question arises about API keys (Google Cloud Messaging):
"AndroidApiKey": "DKJDIiwjerljd290u34jFKDSF"
And it turns out that you need to use a third-party service, that is, Google, in order to simply send a notification about a new message?
Why, why? Is it possible to somehow bypass this "scheme" and not use any third-party services?

Answer 1

No problem, implement in your api a point that gives a list of pushes for the user, and poll with the desired frequency or socket. Or do you think GCM has always existed?