Push notifications is a mechanism to send notifications to the client devices on any event. In Azure, this can be done by setting up a centralized Notification Hubs. Notification Hubs is a service from Azure which provides the capability to send notifications to different platforms on a large scale:

The process starts by registering each platform application with the Notification Service. Notification Service is a system that notifies the client's device on any event. When the device is registered on the Notification service, it sends the handle that the Client App can send to the Backend API App, which sends it to the Notification Hub. When the backend sends the notification, it sends it to the Notification Hub first, and then the Notification Hub sends that notification to the specific Notification Service, which finally delivers the notification to that particular Client App.