Add sections by clicking either one of the two "Add" buttons, as shown below.
This opens a panel listing all the sections available in the template being used.
Upon selecting an item from the list, the section will be added to the project at the location currently visible in your viewport. Flip through the sections to see a live preview in the project and click ADD or CANCEL as you desire.
Choose sections from a different template
While working on a project, you can change the template from which you are adding sections from. Doing this will not change the content already placed on your canvas.
Now, when a section is being added to the page, the list of sections available to add from will be the ones belonging to the selected template.