How to set default notification channel for notification messages that come when an app is in the background? By default, these messages use "Miscellaneous" channel.
As you can see here in the official docs, you need to add the following metadata element in your AndroidManifest.xml within the application component:
<meta-data
android:name="com.google.firebase.messaging.default_notification_channel_id"
android:value="@string/default_notification_channel_id"/>
This default channel will be used when notification message has no specified channel, or if the channel provided has not yet been created by the app.