How to send to "Miscellaneous" notification channel?

As it has been said in another answer, the id of the default channel created by Android System is fcm_fallback_notification_channel, but be careful because the System doesn't create the channel until it has to manage the first push notification. So if you manage all notifications inside your extension of the FirebaseMessagingService class, it may happen that the channel doesn't exist and you run into errors like:

android.app.RemoteServiceException: Bad notification for startForeground: java.lang.RuntimeException: invalid channel for service notification: Notification(channel=fcm_fallback_notification_channel pri=-2 contentView=null vibrate=null sound=null defaults=0x0 flags=0x40 color=0x00000000 vis=PRIVATE)

My recommendation is to check if the default channel exists before creating the notification and create it if it doesn't exist:

private void createDefaultNotificationChannel() {
   if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
     NotificationManager notificationManager = getSystemService(NotificationManager.class);

     if (notificationManager.getNotificationChannel("fcm_fallback_notification_channel") != null) {
       return;
     }

     String channelName = getString(R.string.fcm_fallback_notification_channel_label);
     NotificationChannel channel = new NotificationChannel("fcm_fallback_notification_channel", channelName, NotificationManager.IMPORTANCE_HIGH);
     notificationManager.createNotificationChannel(channel);
}

The ID for that channel is fcm_fallback_notification_channel. The firebase-messaging library creates it internally.

https://github.com/firebase/firebase-android-sdk/blob/076c26db27dd54d809fb2ccff8593b64fb3db043/firebase-messaging/src/main/java/com/google/firebase/messaging/CommonNotificationBuilder.java#L66