How do I send FCM messages to multiple devices at the same time?

Firebase Cloud Messaging (FCM) is a messaging solution that lets you reliably send messages at no cost to both Android & iOS devices . Using FCM, you can send data payloads (via a message) to a device for a specific application. Each message can transfer a payload of up to 4KB to a client.

It’s my first time using FCM. I download a sample from firebase/quickstart-android and I install the FCM Quickstart.

According to documentations, they’re unique , but you can’t bind them to a specific device since they might change.

It doesn’t expire though. It renews itself if one of the following happens. According to https://firebase.google.com/docs/cloud-messaging/android/client: -The app deletes Instance ID.