On a Firebase level there is no connection between a Authentication UID, and a Cloud Messaging Instance ID token.
A Firebase Authentication UID uniquely identifies a user. If that user signs in on a different device, they have the same UID value.
A Firebase Authentication Instance ID token identifies a single app on a single device. If a different user signs in to that app, the app will keep the same Instance ID token.
If you want to associate a UID with an FCM tokens, you have to do that in your application code. For example, many developers clear the FCM Instance ID tokens when the user signs out of their app. This ensures that a new user who signs in on the same device will get a new Instance ID token.
Also see:
Good to know that a different user on the same app on the same device would get the same FCM token, (which proves that there is no connection and explains the importance of deleting the FCM token in logout.)