Systems can be decomposed in many ways. Depending on various aspects the resulting decomposition can look different. If they are divided in terms of technology, you could end up with different compartments or layers classified by the underlying technology. A system can be broken down by structure or function.are a good example. A system can be broken down based on the domain. That particularly helps when a group of people and roles are required to work around the system. Makes it easier for different groups of people to focus on different areas of the system. Hiring, research, experiments all become more manageable.
Another heuristic way of decomposing a system is by identifying the atomics parts of the system. What I mean by that is these sub-units cannot be broken down any further and the behaviour of these units are fundamentally fixed. Either by definition or by some kind of universal laws. By doing this we can then express the system as a combination of these units.
This opens up to thinking about 2 aspects. How can we put these units together in different ways to reach different outcomes. Secondly, how the inherent behaviour of these atomic units can be changed. Has anything changed (new invention, better technology etc.) that can modify the behavoiur of these atomic units. Sometimes it can be easier to solve the problem on the atomic level rather than on the system level.