Firebase Cloud Messaging (FCM) provides a reliable and battery-efficient connection between your server and devices that allows you to deliver and receive messages and notifications on iOS, Android, and the web at no cost.\n Easily target messages using predefined segments or create your own, using demographics and behavior. Target messages to devices that have subscribed to specific topics, or get as granular as a single device.
Set up a Firebase Cloud Messaging client app on Android:
Use link: https://firebase.google.com/docs/cloud-messaging/android/client
After that you have to write cloud functions to send notifications on targeted devices specifically as example
if you want to send a notification when some data is changed in Firebase database like when someone has commented on someones post and you have to send notification to that person that someone has commented on your post then you have to write cloud function accordingly.
for cloud function use link:
https://firebase.google.com/docs/functions
You can look at my app sahayaka for the reference.
Firebase Cloud Messaging (FCM) provides a reliable and battery-efficient connection between your server and devices that allows you to deliver and receive messages and notifications on iOS, Android, and the web at no cost.
IMPORTANT NOTE:
You have to use nodejs for writing cloud functions so if you have no prior idea of nodejs then first of all get some basic idea of it on https://nodejs.org/en/docs/