All elements of an app are included in a collection. A collection is a combination of app basics, dasboards, forms and datasets configured and extended with logic functions and navigation.
After a new application has been created, or when you want to edit an existing application all modules of the app platform will only be available after the selection and activation of a specific app collection.
Deleting a collection is possible in the App Studio - Basics - Main toolbar
You can enable a compact overview of the available collections in the menu part - Profile - Preferences - Studio settings - Compact collection
Selecting "Collections" next to your profile at the top right of the main bar opens the compact panel.
Log out from the App Studio on the top right of the main bar by pressing sign out.
In one (company) app as published in the stores or available on desktop (url) you can have a number of collection, each with his own authorized users and it's specific functionality.