The scope of Firebase Instance Id and Token

I've read a lot of articles about Firebase Instance Id and registration token here, here and here.

I have some confusions about their scope and relationship between Instance Id and Token. I would like to validate that my understandings are correct or wrong.

1. Instance Id is generated per App. Not per installed device. For example, if you have Mail app and Notes App, you will have two Instance Id on firebase.
2. Token is generated per installed devices. For example, if you have ten users installing your app, you will have ten registration tokens.
3. One Instance Id will have many registration tokens.
4. Normally, we only have to keep registration tokens (not Instance Id) on our App Server to send push notifications.