[manigandham]

CPUs do the physical calculation and the number of cores determines the number of calculations that can happen at the same time.

Software threads, from the OS to your application, run on one of these cores for a certain amount of scheduled time, then get switched out with some other thread. If threads are waiting on IO then they're not making much use of the time they get and are wasting CPU capacity.

An older approach is to just make more threads and switch them out faster, but this is very inefficient. Await/async is a way to let threads not get stalled by a single function and switch to a different function in that process that does have work available. It's basically another step of granularity in slicing CPU time within a thread.

The keywords do not force anything, they are just signals to the underlying software that it may pause and come back later if necessary, along with setting up state to track results. Some methods may still run all on the same thread if there's nothing else to do, or if the async result is already available and there is no waiting needed.

Most async/await is usually built on top of yield, generators, promises or other constructs that basically are state-machines or iterators.