The file structure is incredibly simplistic and easy to understand. Separation of language files are encouraged, especially with the hybrid apps developed today using multiple frameworks and libraries.
Model and controllers are developed in back > app
Configurations and routing are in back > sys
Database structure for static databases and seeding back > db
and Composer libraries are kept in back > lib
All routed views are within html > pages
Dynamic views reside in html > template
Build your javascript in js
And your CSS app in css