UIKit
Apple's original framework for iOS interfaces — imperative, mature, and still the foundation under most large apps.
The framework arrived with the first iPhone SDK in 2008 and inherits its design from macOS AppKit, which shows in the naming. You create views, place them in a hierarchy and change them over time by calling methods. View controllers manage the lifecycle, and layout is described with Auto Layout constraints.
SwiftUI has been Apple's recommended way forward since 2019, but UIKit is far from gone. It has capabilities SwiftUI still lacks, and the two can be mixed in the same app. A large codebase built on UIKit is rarely rewritten — it is supplemented screen by screen.