You have to create objects and add them to the list. For example if you want a list of buildings, you can retrieve all the buildings, add them to the list, and return the list.
But if you want a better answer, maybe you can share a brief summery of what you're trying to achieve and some screenshots of your microflow.
The ‘Create List’ action just creates an empty list of the selected object. From your explaination I understand that you just used the ‘Create List’ action and returned it as it is. In order to return something, you need to populate the list first by creating objects of the specified entity and adding it to your created list using ‘Change List’ action activity.
I believe you just want to create mock data for your app. To do so you can create overview and new edit pages for the entity of which you want to create mock data for. The best way to create those pages would be to right click on your entity and Generate overview pages. Add the overview page to your navigation and you can then create data yourself over frontend.
If one want a list of items. Create an entity with a 1 * relationship. For example a building with many departments.
Create this object in a microflow. and then create many items in a mocroflow with each item having an association to the parent building.
This how things are in mendix. In terms of creating the departments alone as a collection, array. Don’t think that’s currently possible to achieve in mendix.