sophia_st I'm not sure I'm following... how does firebase or websockets have to do anything with local notification messages? I was under impression it has something to do with the notifications that goes through some protocol inside google play services, but now I'm lost.
In order for an app to know when you receive a message and therefore display a notification it needs to connect to a server. But if you have multiple apps that are waiting for messages, these connections quickly add up, which consumes a lot of battery. To solve this problem, on a normal (stock) Android phone, instead of every app keeping it's own connection up, your phone only keeps one connection to a Google server for this. The application's server (e.g. Signal's server if someone sends you a message) then informs Google's server about this, instead of your device directly. The Google server forwards this notification then to your device over the aforementioned connection. This way you can still get notifications from multiple apps without needing to keep multiple battery-draining connections up. (Please note that this is only a simplified overview which is missing many details.) This is offered to app developers as a service called Firebase Cloud Messaging. In order for FCM to work, the client (your phone) needs Play Services. If these aren't available apps either can't fetch notifications at all or implement the "classical" approach of keeping up their own connection (that's what I meant with WebSockets). UnifiedPush is an open protocol aiming to reimplement the method used by FCM but without the dependence on Google. Their website gives more information about this, but in practice, this only makes really sense if you have multiple apps supporting it.
But maybe misunderstood what you meant. If you were talking about receiving notifications from a secondary profil while in your main profile: This is possible, but limited. You will see that you got a notification and also from which app, but not the actual content of the notification.