Hat thank you, you made me look at it again. And there I see it, not a library but a service.
com.google.android.gms.analytics.AnalyticsJobService
com.google.android.gms.analytics.AnalyticsService
com.google.android.gms.auth.api.signin.RevocationBoundService
com.google.firebase.components.ComponentDiscoveryService
com.google.firebase.messaging.FirebaseMessagingService
-> Analytucs, sign-in, Firebase and FCM
Thank you for bringing that up.
But when I tested the network activity of WhatsApp some weeks ago with PCAPDroid all it was showing was connecting to WhatsApp servers once every 30 seconds presumably providing notification service. That is without sandboxed Play Services installed.