trying to modify magento theme blank / luma i realized that code with name -> app/design/frontend/magento/theme/web/css/source/lib is not available in the application, but referred many times in other files mainly .less code.
an example is less code contained in: app\design\frontend\Magento\blank\web\css\styles-l.less
in line 30 displays:
but no folder /lib or file _responsive.less is available.
any guidance appreciated.