firebase admin is only available for admin or users also to send notification to other users?

Question

Firebase admin is only available for admin or users also to send notification to other users? Actually i am little bit confused in send notification from one user to another and which data is used like on the basis of user receiver uid or receiver fcm token.

Answer 1

firebaser here

Calls to the FCM API to send messages require that you specify the FCM server key in your code. As its name implies, this key should only be used in server-side code, or in an otherwise trusted environment. The reason for this is that anyone who has the FCM server key can send whatever message they want to all of your users.

So sending messages to users is indeed only possible from a trusted environment, either through the Admin SDK, or the REST APIs. See How to send one to one message using Firebase Messaging for more.