I am thinking about the challenge to software developer and am wondering who is scheduling all these general software codes to different processing components of the machine? As software developers, what can we do to let the scheduler schedule our program onto computing resources more efficiently?

I have written quite a lot of iOS application code before, and I'm wondering how the OS/device decides to run certain parts of the code on special processors while running other parts on the generic-purpose CPU. Does this work assignment happens at the API level (e.g. some functions in NSFoundation library would be run on a specialized processor) or does it happen at finer granularity (e.g. compiler recognizing the code pattern in a snippet and mark the work for assignment onto special cores)?