User-specific checklist data is not isolated. changes made by one user are visible to all users.

developing an app that acts as a checklist for video games, helping users track their progress to achieve 100% completion. The app includes checklists for story missions, side missions, etc...

I have set up Firebase collections for users, story missions, side missions etc...
Initially, all users should see the same default checklists. All the checkable elements are already created in the database, and users cannot create them themselves, they can only mark items as done or not done.
However, I want each user to see their own list and only see changes they make. Currently, when I make changes as one user, those changes are visible to all other users when they log in. I need each user's changes to be isolated so only they can see their updates.

Subcollections : I tried using subcollections for each user. This worked to an extent, but new created users have empty subcollections, and nothing is displayed.

App State: Attempted to use App State to manage user-specific data, but this approach didn't resolve the issue.

Generate Dynamic Children: Tried to generate dynamic children, but this method also didn't work.

Document References: Tried to create document references to users in each "checklists" collection and also create document references to the "checklists" collection within users, but neither approach worked, and I'm out of ideas.

Please note that I am new to Flutter, Firebase, and databases in general.

Also English is not my first language, so I apologize if the phrasing is not perfect.

Thanks !