In your case, you might consider having your own app server that will provide an API for your angular app to send notification. Here’s a documentation on how you can send a notification to a device from your server.
https://firebase.google.com/docs/cloud-messaging/send-message
There are different parts of this implementation actually. First, your app server needs to provide an API to the Android app that will be used to send the firebase registration token from your app when the app is launched. You might consider storing those registration tokens in your backend database.
Then the other API that your server will provide, to send notification, can be integrated in your angular app. The server API should be able to find the exact registration token of the device where you need to send the notification and will call the API from FCM to send the notification to the device. Here’s the API doc that you might consider taking a look into.
https://firebase.google.com/docs/cloud-messaging/http-server-ref
Here is a good tutorial that shows how you can implement the server side implementation to send the notification.
https://www.pluralsight.com/guides/push-notifications-with-firebase-cloud-messaging
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