Outershell is part of solid tools, but actually a separate menu item in Tools menu just above " solid tools" It would make a combined “solid” element of all the faces and edges in one or more components. But those components (or groups as well) have to be “solid” to begin with.
I use the intersect command a lot with joining roofs. I also do as you noted, for some gable roofs. The two sides are mirrored instances of the same component. Sometimes I leave them as they are without combining or trimming until later, adding the same features (such as gable details) that will be duplicated in each side of the roof. As the plan develops, and specific features are needed, I’ll make them unique and modify each component or combine them.
I do not often use the solids approach for this, but that is a way to keep the geometry all together and clean–and you can later subtract and join to it as if it were a solid piece of material.
DC components are specially designed components that have parametric features. You can make them or use components designed by others. I think some examples were given above that did not show properly.