How to access other users display names

[AdrienLemaire]

I use firebase_ui_auth screens, and users can set their display name this way.
But I couldn't find a way for users to access other users display name through firebase auth.
So I added a firebase function to copy each user data in firestore on account creation.
Now, when they change their display name, the display name is not updated in the firestore entry. And UserBuilder doesn't have an onUpdate trigger.

What is the recommended way to fetch a user's display name from his user id when it's not the currently logged-in user ?