Push Notifications are messages that are sent directly to the user’s mobile device.
App Publishers can send them at any time and the message will pop up even if the device is locked or if the user is in another app. They can be used to provide a richer experience to the application users, like sharing updated information or events.
Applications don’t have control over sending push notifications to the user as they are managed by push notification services like (Firebase Cloud Messaging).
These services uniquely identify devices with an identification token and not logged in users of the apps. They then send this token to its own service which will be used to send messages to the device in a specified format.