C++ - how does Sleep() and cin work?

The OS uses a mechanism called a scheduler to keep all of the threads or processes it's managing behaving nicely together.

several times per second, the computer's hardware clock interrupts the CPU, which causes the OS's scheduler to become activated. The scheduler will then look at all the processes that are trying to run and decides which one gets to run for the next time slice.

The different things it uses to decide depend on each processes state, and how much time it has had before. So if the current process has been using the CPU heavily, preventing other processes from making progress, it will make the current process wait and swaps in another process so that it can do some work.

More often, though, most processes are going to be in a wait state. For instance, if a process is waiting for input from the console, the OS can look at the processes information and see which io ports its waiting for. It can check those ports to see if they have any data for the process to work on. If they do, it can start the process up again, but if there is no data, then that process gets skipped over for the current timeslice.

as for sleep(), any process can notify the OS that it would like to wait for a while. The scheduler will then be activated even before a hardware interrupt (which is also what happens when a process tries to do a blocking read from a stream that has no data ready to be read,) and the OS makes a note of what the process is waiting for. For a sleep, the process is waiting for an alarm to go off, or it may just yield again each time it's restarted until the timer is up.

Since the OS only resumes processes after something causes it to preempt a running process, such as the process yielding or the hardware timer interrupt i mentioned, sleep() is not very accurate, how accurate depends on the OS or hardware, but it's usually on the order of one or more milliseconds.

If more accuracy is needed, or very short waits, the only option is to use the busy loop construct you mentioned.

The operating system schedules how processes run (which processes are eligible to run, in what order, ...).Sleep() probably issues a system call which tells the kernel “don't let me use the processor for x milliseconds”.

In short, Sleep() tells the OS to ignore the process/thread for a while.