If you have objects you'd like to hide in either mobile or desktop view, click the object, then click the Visibility icon in the Properties tab.
You can also control visibility by clicking Contents Panel on the lower left of the Page Builder.
When you select objects within the Builder, the Contents panel will automatically highlight and scroll to that object. You'll see an eye when the element is visible or a solid circle if it's not. Click to toggle between the two.
NOTE: If you opt to hide a box or section with objects nested within, all nested items will be hidden.
Moving Elements Between Sections and Unparenting Nested Elements
By default, if any element is moved between sections, it will be moved in both views. To move elements outside of a section without changing both views, hold down the Command key (Mac) or Control key (Windows) while you drag an element. You'll see a reminder of this anytime you move an element from one section to another without holding down Command or Control.
If multiple elements are nested in a box, moving those elements in one view, will also move all elements in the other view. You can also use Command/Control+click to unparent an item in one view and move it independently of the other view.
Remember that you can also Shift+Click to move more than one element at a time.
You'll see a warning if any of your objects are out of the bounds on your content. Just move the object within your page boundaries, or hide it, to remove the warning. You can also disable out-of-bounds warnings using the checkbox on the upper right of the Builder.