OS ( Specifically Android's ) Process

Question

A book I am reading says

Each process has its own memory space, separate from the space belonging to other processes. Processes interact with one another only through narrow, well-policed pathways.

And then goes on to say

The system schedules the running of processes. To do this, the system executes a sequence of statements in one process, then a sequence of statements in another process, then a sequence in a third, eventually returning to the place where the first process left off.

My question is why are processes scheduled to run from one to another to another back to the first. I looked up more info about processes in the docs under Processes and Threads and it helped some, but I still don't see how they go from A -> B -> C -> A unless it is like the stack of functions in C and C++. I read the part on process but still don't know 100% what they are and I was lead to the Services and content providers section onf the doc becuase it talks about them in the Process section about the priority of processes components and it's life cylce. I don't know if the priority of the components are related to understanding how it goes from A - B - C - but I think I need to know more than the definition of proccess as

"A typical operating system (Android included) has users, processes, and threads."

and

"Each process has its own memory space, separate from the space belonging to other processes. Processes interact with one another only through narrow, well-policed pathways."

Answer 1

For background information, I'd recommend reading Wikipedia's articles on Process (computing) and Thread (computer science). Also possibly useful (though slightly dated now) is the chapter on process scheduling from Understanding the Linux Kernel.

For an Android-specific answer, there is a previous question on Android Process Scheduling that has several answers.