Well you are right in some sense android should take layout dependent on different densities but some mobile do not fall under specific density. Therefore android will pick up default layout from layout directory.
to support multiple screen resolution provide different layout for different screen sizes, you can make following directories in res
directory like this
layout-hdpi
layout-mdpi
layout-xhdpi
layout-xxhdpi
layout-w320dp-h408dp
layout-w480dp-h800dp
layout-w480dp-h854dp
layout-w720dp-h1280dp
layout-w1080dp-h1920dp
layout-w1440dp-h2560dp
when you provide layout in all this directories you will give multiple screen support for different sizes as well