An aircraft is a complex system. In the design stage and in the flight and maintenance manuals (used by pilot and maintenance technicians) it is broken down into simpler systems that carry out homogeneous functions. Some examples include but are not limited to:
Controls and displays in the cockpit panels are usually grouped by system.
Introduction to Aircraft system is one of the subjects for the degree in Aerospace Engineering at the University of Bristol.
Aircraft systems and their operations are described in the Flight Manual of the Aircraft (the so called -1 in Military slang) in part 1.