If you create a blank new project, you’ll be presented with your Home (index) page first. This is a page that cannot be deleted, and represents the starting point of your app. Your starting point is 375 pixels wide by default.
You can create new pages inside the left “Main” menu by clicking on the page button.
The pages are adjustable and show you the width and height of different devices on the right bottom corner while resizing it.
In order for your app to dynamically stretch to different device sizes you need to apply an auto layout and constraint properties which you can learn more about in the following articles.