When we talk about complex systems we consider something like “rocket science”—and the systems in the space industry are complex indeed—like control rooms, cockpits of spacecraft, and so on.
But other examples are related to normal Earth-located objects, like power plants, vessels, traffic control towers, oil refineries, and airplanes.
Most information systems in business and financial accounting are complex too. The same is related to medical systems and equipment, scientific research, construction and production facilities, etc.
Moreover, the software that is used in information technologies, like IDEs, digital and computer-aided design (CAD) systems are also quite complex.