Looks like you're getting a bit confused with handles, specifically how Magento utilises them.
You referred initially to social_index_index.xml. This is actually loaded because in layout there was a handle added -
social_index_index. This would have come from the auto generated handle formed from the route name, controller name and action name that was then added as a layout handle (this is formed here and actually added as a handle here).
What you're looking at with catalog_category_view_type_default.xml is another layout XML file that would be loaded when
catalog_category_view_type_default is a present handle. This would happen in a category that is not an anchor (no layered navigation) and would have been caused by this code in the category view controller.
Another good example to demonstrate this is the
default handle. How come
default.xml layout files always apply? Well, Magento leverages handles just like both examples above. You can see the handle added here
So handles will load layout XML files with the same identifier (social_index_index -> layout files named social_index_index.xml apply). You could add your own handles (there are many way and places you could do that) and that would get included (catalog_category_view_type_default added by some logic -> layout files named catalog_category_view_type_default.xml apply)
This explanation skips over alot but you seem to have at least a basic understanding of layout so hopefully this all makes sense to you!