You can select elements of the project by clicking on them on the Floor plan or in the 3D view. By pressing the CTRL or the SHIFT button you can select more elements. This is the basic way to select elements in the project. To select elements with a more advanced tool, click on the Selection tool.
Location: Ribbon bar > Edit > Select
Here you have the option to select elements within or out of the selected area on one floor or on all floors. You can narrow the list of selected elements by pressing the By common part button or the Subtract button.