Subcomponents would be the components that are contained within a nested component. Work from the bottom up, not the top down as you are describing. Make more than one component and select them. Create a component that contains them. Now you have a nested component at the top level and subcomponents inside it.
Here’s an example of a nested component with subcomponents inside. The drawer is the nested component and the parts that go into building the drawer. Note how it is shown in Outliner.