From concept to functions

I don’t have a background in professional software engineering or development but I do enjoy writing small personal projects in Python. I’m really interested to know, let’s say a company or individual entrepreneur says ‘my concept for an app is x,y,z’. Then my assumption is, the high level goals for this project would go on to a Kanban board or similar. Then a developer or a team of developers would each look at the goals and then work on that component. My questions is, is there ever a definition of functions, for example ‘in order to achieve this goal, we need a function that accepts this kind of input and return this kind of output’ and then the developer just needs to comply with that design? Or does the developer have to come up with the inputs and outputs themselves. I know my question is quite conceptual but I’m interested to know how it works in the real world.