You can create multiple Artboards to change the layout of your ad templates. This can be done by adding a new one, or cloning an existing one.
To add a new Artboard, hover on the "+" icon at the bottom of Ad Builder.
This will bring up size options for your Artboard. Choose the one you want to work with. These options will change depending on the platform you choose in this step.
Variant layers will appear on the new artboard.
Select the "Clone" button on the top right corner of the Artboard to duplicate it. The new, cloned artboard will retain all the layers from the original Artboard.
Select the "Trashcan" icon on the top right corner of the Artboard to remove it.