UIKit is the iOS framework that implements the standard UI components for iOS applications.
Building an app with UIKit is trivially easy. In all cases you'll use Xcode, Apple's IDE for developing for iOS and macOS.
In Swift, you just put the statement
At the top of a each source file in your iOS application that uses the UIKit framework.
In Objective-C, your import statements should be prefixed with a
#, and you need to provide both the name of the framework and the name of the header file that you are importing. For UIKit, the import looks like this;
(The angle brackets around the framework name and filename tell the compiler to look in the project's frameworks directory rather than the project source directory.)