Components of the structure can be any arbitrarily chosen parts of the hierarchical structure. Complexity evolves from a simple working system. It starts as a small module, then a collection of different small modules are added. These modules are then combined in different ways to do complicated things. Then the groups of modules are combined, with other groups of modules, to do even more complicated things.
Complexity is something which is built up in easy steps from simplicity.
The break down of complexity is through the concept of class and object. Different combinations of instructions will produce different variations of an object. A few different objects working together in different combinations will do a multitude of different things and a few different instructions combined in different ways will give a multitude of objects. Thus from a relatively few basic elements a vast complexity can be constructed. To my surprise I received a whole batch of private email responses to this post.