Well for a messages app it either needs a background service polling the server for new messages or it needs to use Firebase Cloud Messaging. AFAIK, FCM is esse

Well for a messages app it either needs a background service polling the server for new messages or it needs to use Firebase Cloud Messaging. AFAIK, FCM is essentially a service running on your phone that receives messages from Google's servers and forwards them to your apps. And backend servers can send messages to those Google servers.