Those are created as additional pages in Store Builder. The StoreFront Help Content’s Stores Builder (link) page doesn’t make this particularly clear, but if you compare the storefront image to the page folder tree shown on the Page Manager sub-topic you can see how additional pages appear as links in the page’s navigation bar - if additional pages are “nested” under any given page, then they will appear in a dropdown menu box under the ‘containing’ page’s link in the nav bar.