For open source messengers, you can check whether they actually encrypt your messages and whether the server has access to your encryption keys but what about WhatsApp? Since it’s not open source, you can’t be sure that the encryption keys aren’t sent to the server, right? Has there been a case where a government was able to access WhatsApp chats without reading them from the phone itself?
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.