A windmill is a simple system. The wind turns the blades of the windmill which spins a generator which produces electricity. The windmill is a self-contained system which has inputs (wind), process (the windmill blade and the electrical generator) and an output (electricity). Therefore, by definition, the operating windmill is a system.
However, systems do not exist in a vacuum. Generally, two or more systems operate in conjunction, taking the intermediate products or outputs of the individual systems and producing a larger, integrated product. We call this combination of symbiotic systems a macro-system, or system of systems.
Consider a gasoline engine manufacturing company. It takes a variety of raw materials and parts, performs a manufacturing process on them, and sells completed gasoline engines. These engines can be bought by consumers, or, they can be purchased by automobile companies, yacht builders, electrical power generation utilities, and a myriad of other businesses which incorporate the gasoline engines, via their manufacturing or production process, into their products or outputs.
The combination of systems -- the engine building system, the auto building system, the auto sales system, and the ultimate driver make up the macro-system.