You have to create and Firebase cloud functions acording to this document: https://firebase.google.com/docs/functions/get-started. After that you can use this index.js file to send notifications to all users by subscribed topic groups.
You have to create and Firebase cloud functions acording to this document: https://firebase.google.com/docs/functions/get-started. After that you can use this index.js file to send notifications to all users by subscribed topic groups.