… I guess you are talking about what called in Sketchup : component ( or group. )
SU importing your dwg as a component. Inside this component there are individual edges, (curves and arcs are also there, but they are really a series of edges) and other sub components. Even in some sub components there are other level of sub component.
What you can do:
Double click ont he component to enter inside the component context and edit the geometries. When you done: Left click outside of component or Right click “Close Component” to close it.
Right click ont he component and select “Explode” therefore you will “destroy the container” and you can edit directly
In both case as mentioned there will be an sub-components inside what you can handle similar way as the “outside” component.
Here it is described more precisely and detailed :