FCM (Firebase Cloud Messaging), and Apple's own APN (Apple Push Notifications) work as a single OS level service, and they are multiplexed so that many different apps can route through them with a single connection, even waking your phone/watch from sleep. There are open/de-Googled alternatives like UnifiedPush, but they're not integrated at the OS layer (they run as a background worker thread), and can't match the battery efficiency or sleep-wake capabilities of those built into the OS/low-level networking layer.