The dotx file that stores built-in building blocks isn't empty. There is no text to read, and there is no user interface that I know of to allow you to view the content, but it is there. It is the file at the first path you listed.

Custom building blocks can be saved in any Word template that is .dotx or .dotx. But that 2nd path you listed is available as a save location for custom building blocks that you want to be available globally.

I don't know of a way to actually see anything in these files as they don't contain text.