Ever noticed how those notifications include the message content and the sender? Google has access to this information, despite the encryption.
Not necessarily. I work on a messaging app, and we only use firebase to “wake up” the app. Initially the notification doesn’t display anything meaningful, but the app very quickly connects to the server (tells the app who it should connect with) and then the peer (to finally get the actual content). The notification is updated once we have the content. But it typically goes so fast that you only ever see the final version of the notification.