Now that we've taken care of the important aspects of persistence using <b>Firebase Authentication, Database, and Storage</b>, we can look at building out our main UI. To start off, we'll look at how the Instagram user interface is designed. Then we'll start the process by including our own custom <b>UITabBarController</b> subclass. Once we have the main UI set up, let's also fetch the user's username and display it at the top of our <b>UINavigationBar</b>.