fcm
FCM says delivered, the phone shows nothing: the data-only message
An FCM message with only a data block is never drawn by the system — your app has to build it, and FCM reports success either way. Why, and what to send instead.
android articles
Everything we have written about android.
android
Your Android notifications are being dropped by a channel you never created
Since Android 8 a notification naming a channel your app never created is discarded by the system, and FCM still reports it delivered. How to confirm it.
ios
You get one notification permission prompt. Most apps spend it in the first ten seconds.
iOS and Android each give an app one permission dialog per install, and a decline is close to permanent. How to ask before you ask, and iOS provisional auth.