The biggest change is with a new Focus system, which will allow you to customize how your iPhone, iPad, and Mac all function when you need to concentrate on a particular type of task. The iPad has seen a significant change with the ability to place Widgets on the home screen. This will create a whole new experience, particularly with the extra large widget size.
For developers there are significant changes with Swift’s new async/await pattern, a whole new button system for customizing buttons, and a new continuous integration service called Xcode Cloud. Xcode itself has some improvements like inline commenting and the ability to review, merge, or close pull requests.
SwiftUI, Apple’s Swift-only UI framework has some enhancements with a new refreshable modifier, new button options, a new primitive called AsyncImage.
These topics just barely scratch what is covered in iOS 15, iPadOS 15, tvOS 15, watchOS 8, and macOS Monterey. The information within will provide details about all of the new features of each operating system and has something for everyone, no matter what level of expertise.