Yes users with the right credentials can add their own objects to a published App. They can decide to publish them to other users of the App or not.
Any changes to master items would be done in the base app. These changes are made by the owner of the App and are applied to all users of the app when published. Note modifying the base app will not effect other users objects unless the change made is to remove or change an existing master item currently in use by other users.
1. assuming we have few subsidiaries in our organization and we are building an overview app. i wish that the would be available for ALL General mangers and group's management as well BUT - i wish that the main sheet (main KPIs) will present data based on the user role (i.e. while users from group management will see as default KPI for the whole organization users from each subsidiary will see the same KPIs but filtered according to the relevant subsidiary). is it possible? i don't want to reduce data but only create few default states for the main sheet.