Imagine if you got tasked with building an airport but you’ve never built an airport. Well, you’d have to first clarify requirements: What kind of planes? What’s on these planes? What kind of cargo? How many flights per year?
As you learn more about requirements, you need to start solving them, but remember to keep it at a black box level. Let’s say your airport will be moving both cargo. How will cargo come in? How will cargo get out? We probably need a “cargo movement sub-system.” A cargo conveyor sub-system is still a black box — you don’t yet know details but you do know, at this very early point in questioning, that you need “a system that moves cargo between the loading bays and the plane.” You don’t even know if it’s going to be a conveyor system or humans manually driving cargo yet. However, you are well on your way to building an airport system.
You keep at this and keep delving down deeper until you go from design to implementation. This is the process of how you build anything in any field.