Admin CP and Sub-Directory

At the second point in this guide https://philsturgeon.uk/blog/2009/07/Cre...deIgniter/ talks about the sub-directory for create an admin cp and the routers.
But my question is: How create a general route for every controller that I create in the sub-directory "admin" for example?

And when I write $route[‘admin’] = ‘admin/dashboard’; codeigniter should not look for the controller "admin" with the method "Dashboard"?

Sorry for the questions, I have probably done a bit of confusion.
Could you explain how to create an admin cp using subdirectories?

Thanks for the reply.