While editing the desktop version, the mobile version is automatically generated. You can add specific elements to the mobile version, in most cases, it is because you may not get the same outcome as you had hoped for compared to the desktop version.
Start by going to the mobile version by clicking the mobile icon on the left side of the editor. This will then display the mobile version of the template.
From here, go to add area and drag in a content block and edit it as you please. If you then go back to the desktop view, you will notice that the element you added does not show in the desktop version, only the mobile version.