I'm trying to grasp the concept of an operating system at the moment (on Unix-like machines)
The kernel is the process with PID 0. Of course the Kernel is not really "just another process" because a process is a concept of the kernel, but it's not like the kernel yields control to some other component (...?).
Everything the kernel can do, can be looked up in the source code of the kernel (e.g. the Linux kernel is on Github for everybody to see)
So the operating system starts with the kernel as the active "process". It then yields a time slice to some other process (e.g. the process with PID 1), but at the same time assures, that it will give control back to the kernel.
So the control of the CPU is always: kernel -> some process -> kernel -> some process -> (...)
Never: some process -> some process
Systems calls are an API for userland processes to do some things the kernel is capable to do. All "advanced features" (accessing files, connecting to the web) of any program can be traced back to some system calls.
Do I have any grave misunderstanding?