Which layer of Apple's operating systems hierarchy is considered the most complex?

The most complex layer of Apple's operating systems hierarchy is indeed the application layer. This layer is where end-user applications reside and interact with various system features. It encompasses a vast array of functionalities, including not only the user interface but also intricate behaviors, functionalities, and integrations with other services. The complexity arises from the need for these applications to handle various tasks, systems, and adapt to different hardware configurations, user input, and networking capabilities.

Additionally, the application layer must interface effectively with other layers below it, making use of the frameworks and services provided by the framework layer and the capabilities exposed by the kernel layer. Each application may utilize various frameworks to fulfill different tasks, complicating the overall architecture and requiring careful design and implementation.

In contrast, the kernel layer primarily deals with managing system resources, processes, and hardware communication, while the device layer focuses on drivers and direct hardware interactions, which can vary in complexity but are less user-facing. The framework layer serves as a set of tools and libraries that streamline the development of applications, but it does not have the same level of complexity regarding user interaction and functionality as the application layer does.