Structure Layers and Subitems

New to Monday and our agency is still rolling out Monday. Is there a simple way to explain the layers to organization or a graphic for Workspaces/Boards/Projects and how they should be layered? Is there a best practice or structure your agency has found?

We also have a project board that we would like to import a test plan from excel into Monday. The columns we want for testing do not match the rest of the board. Is there a way to import an excel spreadsheet as subitems? Can we create a separate board that is attached to the original board for testing?