From the field section on the left side of the app designer, you can either click on the plus (+) sign that appears when you hover over the field or you can click and drag the field into the layout section.

When using the plus sign to add the field, the field is added to the bottom of the app layout below all other fields. The field can then be clicked and dragged to be repositioned elsewhere in the layout of your app. 

When clicking and dragging from the fields section the field can be placed anywhere within the layout of your app. Including nested within a section or repeatable section.