Push-notification for mobile apps are having most important feature. This is most favourable mobile engagement technique .It’s simply a matter of how you strategize and market your product using Push Notification. Push Notification offer marketers a greater opportunity to engage with customers. Resources should be funnelled into executing push messaging campaigns.There is always a question for new bees in Mobile development arena how Push Notification has to configured in your application.
Below approach are quite simple and useful where most of apps & their campaign portal utilise it
Overall Picture

Front End team needs to register device Id & Topic Against Each Device
For Individual Device Message , Firebase Admin SDK API needs to build which will be triggered via Admin Portal or some daily cron job depends on your campaign requirement .
Example : On Successful payment received, SDK api will be called with Device Registration Id of user along with Message Payload. API will send this Notification to specific user
For Generic Message to All, Admin Portal system needs to build Send Notification framework which will trigger Notification to topic against which all devices are registered
OR any content management portal can call same NodeJS API with Message Payload & Topic Name
Example : New Promotional Notifications or New Year Greeting to All etc.
This is 2 step application setup.
Front End Work : this has to be implemented in iOS or Android
Every Device has to be registered with FCM or SNS Framework
During Registering process we will be getting registration ID
registration ID has to be stored in backed with conjunction of UserId
Every Device should also be registered against one Topic as well
Application code on mobile for receiving Notification and displaying it
Topic – will be used for sending message to ALL device
Registration ID- will be used for specific device
BACKEnd Work :
Create one API End point using FireBase Admin SDK
Using the SDK, End point will have inputs either
Topic with Message Payload
registration ID with Message Payload
For more:
Firebase:
https://firebase.google.com/docs/cloud-messaging/admin/legacy-fcm
Node JS Admin SDK :
https://www.npmjs.com/package/fcm-node