How do CSP streams work?

Question

C++ / C#

There is such a library for C libmill - Go-style concurrency in C , how does it work?
What is the difference between CSP co-programs and threads?
On the example of this lib and in general, an approximate algorithm for calling a function as a CSP?
How will threads/csp behave when there is only one processor? I understand correctly that the thread will receive time from the system scheduler, like the main program, and although this is not parallel, it looks very cheerful and fast on the outside? How does it work in CSP?