The kernel plays an essential role, as it manages how your phone, tablet, and PC's hardware and operating system.

Applications "talk to" the operating system for all user interface and file management operations. Also called an "executive" or "supervisor," an operating system performs the following functions.

An operating system is a collection (or suite) of programs that manages and controls the computer. Operating systems have many functions: ...

This IDC survey of 1,025 IT decision makers illustrates cloud strategies at organizations and the role of server operating system environments. Commercial Linux is the only environment that surveyed ...

I think the diagram is just trying to emphasize that operating system functions are usually invoked through a different mechanism then a simple library call.

A kernel can be defined as the essential center of an operating system. The kernel is the core of an operating system that provides a set of basic services for the other parts of the operating system.

The course covers nine areas, with the first six being core operating system functions and the remaining three covering more advanced operating system concepts.