How Firebase push notification work on IOS?

I want to create my own push notification mechanism for my own iOS applications.

I compared some services like pushy.me or Google Firebase. I think Pushy is the only service that can push notifications independent from Apple APNs, but it uses simple HTTP long-poll requests to receive notifications (in the iOS SDK).

But how does Firebase work? Does it still depend on Apple's APNs? How will it affect my battery life?

Firebase Cloud Messaging (FCM) delivers push notification to iOS devices via Apple's Push Notifications service. Also FCM extends functionality of push notifications. How FCM extends?

• FCM works with iOS and Android. Cool feature if you have the app for both platforms;
• Don't need to develop backend for sending notifications, storing pn tokens etc. Just register your app in the Google Developer Console and follow User Guides. For sending a push just execute request to https://fcm.googleapis.com/fcm/send with params;
• Broadcast notifications. The app subscribes to a topic and then you can send a notification to all topic subscribers. Very cool;
• Upstream messages (send data to the server)

Also Google has others services you can extend FCM with. For example Cloud Functions.

I didn't find that FCM integration take big affect to battery life in my apps.

UPDATE: FCM framework sends push notification token (and other info) to Google services. Also as I mentioned above you can subscribe app for a specific topic. Than Google knows which device needs to send a push to. There is a possibility to setup FCM in iOS automatically(with using method swizzling). FCM exchange AppDelegate methods and knows your's device pn token.