What are /r/PushNotifications' favorite Products & Services?

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This blog will help you understand how push notifications works, it’s concepts and application. Do give it a read and let me know if it helped! https://www.applozic.com/blog/native-push-notifications-the-concept-applications-and-considerations/

I agree, But I believe they provide the complete product to the buyer. So that you can customize it as per your need. Still, if you are not looking for anything like this then it's better to take reference from FCM documents.

https://firebase.google.com/docs/cloud-messaging/