This also should never be forgotten. Any human or animal group can be described as composed of parts. But we should always carefully remember that such description must be considered merely as a useful artefact. Let us remember that H2 and O, when we study H2O, are H2 and O, plus their chemical bonds, their relationships, within the water molecule. As systems can exist only in time, processes are the other indispensable part of our description.
Processes are the changing aspects of structures and relationships within an active system.
Still, no system is ever isolated. Thus its interactions with other, external, entities must be explored and understood. Structures and processes are of many different kinds. We thus end up with a corresponding number of different models. Growth, for example, can be lineal, exponential, asymptotic, logistic, cyclical, etc.