Think of things like in real life. a side of a TV belongs to that TV and has nothing to do with a cabinet next to it They’re all “groups”. In SketchUp you have entities as edges and faces. On their own they are basic entities, But put together they may represent something, although maybe not finished. This is a good time to group them. You can edit then later anyway,
Groups (and components) are chunks of geometry separated from the rest Their geometry does not stick to geometry, nor does their edges intersect with edges in other contexts (say in other groups or in the topmost context/ungrouped)
You’ll notice that till now I haven’t mentioned layers. All basic geometry (except for text and/or dimensions) must “stay on” Layer0. Only groups and components may have another layer assigned to them to controle their visibility (by layer o/off).
You say your exterior is already grouped. Select this group and in ‘Entity Info’ assign some other created layer to it. Now you can turn its visibility off. But only due to the grouping it is separated from the rest, not just because you made it invisible.
Ungrouped invisible geometry still merges and intersect and stick to visible basic geometry.