Firebase Firestore - relational data design approaches

I'm totally new to Firebase, and I'm trying to get my head round the best db model design for 'relational' data, both 1-1 and 1-many.

We are using the Firestore db (not the realtime db).

Say we have Projects which can contain many Users, and a User can be in multiple Projects

The UI needs to show a list of Users in a Project which shows things like email, firstname, lastname and department.

What is the best way to store the relationship?

• An array of User ids in the Project document?
• A map of Ids in the Project document?

Ive read the above approaches were recommended, but was that for realtime database? Firestore supports Sub Collections, which sound more appropriate...

• A sub collection of Users in the Project document?
• A separate collection mapping Project id to User id?
• A Reference data type? I've read here https://firebase.google.com/docs/firestore/manage-data/data-types about Reference data type, which sounds like what I want, but I cant find any more on it!

If its just a map or array of Ids, how would you then retrieve the remaining data about the user? Would this have to sit in the application UI?

If its a sub collection of Users documents, is there any way to maintain data integrity? If a user changed their name, would the UI / a cloudFunction then have to update every entry of that users name in the Sub collections?

any help / pointers appreciated...