The world level is for creating terrain, buildings, and any design elements not linked to a specific building. You can switch to the World Level by clicking on the World tab in the navigation bar.

When you create a design, by default an empty building is created. To create another building, switch to the World Level. Then click on World Setting icon in Global Preferences and Settings bar at the bottom left.

Then go to Add Building in the panel. Click on New Building to start a new empty building.

To clone a building, click on World Setting icon in Global Preferences and Settings bar at the bottom left while on World Level. Then go to Add Building in the panel. Click on Copy Building to copy a building. Then click on the name of the building you want to clone.

To delete a building, click on World Setting icon in Global Preferences and Settings bar at the bottom left while on World Level. Then click on Delete Building in the panel. In the new panel, click on the name of the building you want to delete.

To add objects/elements in your design which are not linked to any specific building, you can add them on the World Level. First switch to the design level, and here you can add your objects. This could be boundary walls, roads, trees, or any other landscaping elements as well.
These objects/elements will not be visible when you are on the building or floor level, but will only be visible on the World Level.