The Sketch file is well organized, labeled, layered and full of editable shape layers. Most people find it useful for mocking up apps. Others use it to concept ideas or create custom interface elements that work harmoniously with those native to iOS. Whatever your use, we hope you enjoy it.
Before you get started, you’ll want to become acquainted with some of the support documentation that Apple has put together over at developer.apple.com (login required). PDFs like the iOS HIG, UI Transition Guide and UI Catalog will help you become familiar with how and when to use these elements.