If you write your code in the right way they don’t have to. That’s the point.
You shouldn’t need to comprehend your entire program at once to work with it.
> Even with best practices, FP, whatever, function composition alone can’t always elevate the complexity to the requirement’s level
Function composition isn’t the pinnacle of abstraction. We have many other abstraction techniques (like rich types, lawful typeclasses, parametric programming, etc.) which allow for any given subcomponent of a program to be conceptualized in its entirety.