I don't know too much about this, but is this not APNs [0]? I see there is a FCM integration (which is how I assume FCM does it anyway) [1] as well as a standalone Flutter implementation [2].
How would iOS native only apps on Swift do push notifications, surely Apple has their own service and everyone doesn't use Firebase?
[0] https://developer.apple.com/documentation/usernotifications/...
[1] https://firebase.flutter.dev/docs/messaging/apple-integratio...