The directory structure for the project is done in a way that allows it to be built in Android Studio.

The direct file structure seen in Windows explorer and such is not representative of the development file structure.

Once in android studio, Strings and XML files for creating the UI of activities go into the res folder into their appropriate directories.
ex:  app/res/vaules/strings.xml is where you would put text resources for the app to draw upon
ex2: app/res/layout/activity_login_theme.xml is where we setup the UI layout for the login theme page of the app

General files about the app should be placed near similar files about the app, following the example set by previous developers.

For example, if a file that is being added onto lies in app/java/com/owncloud.android, the additions should also lie in that general directory.

For more information on how the directory structure works, refer to the official nextcloud GitHub and see examples as to how they have added onto their own app. ( https://github.com/nextcloud/android )