APNs

Your server opens an HTTP/2 connection to APNs and sends a JSON payload (up to 4 KB) addressed to a device token. It authenticates with a signed token (a JWT) made from a .p8 key in your Apple Developer account, which works for all your apps, or with older per-app certificates that must be renewed every year. There are separate sandbox and production environments, and a token from a development build only works with the sandbox.

APNs also carries silent background updates, incoming call alerts and Live Activity updates. Apple limits how often background pushes wake an app, to protect battery life, and users control alerts for each app in Settings. Services such as FCM and OneSignal still hand iOS notifications to APNs behind the scenes.