This document is based on developing IOS using the storyboard. I have not gone on to developing with SwiftUI yet.
Definition: Application development environment for IOS which includes two core frameworks: Foundation and UI Kit.
Defintion: Framework that grants you access to essential data types, collections, and operating system services to define the base layer of functionality for your app.
Definition: Framework that constructs and manages a graphical, event-driven user interface for your iOS or tvOS app.
Definition: UIViewController is a UIResponder object that manages a view hierarchy for your UIKit.
Read the UIViewController document for additional explanation.