Themes Path Map configuration

This a documentation question. How does the Themes Path Map settings work? There are some predefined entries,

"Path Mask" => "Alias"

@app/themes/*/modules => @app/modules
@app/themes/*/widgets  => @app/widgets
@app/themes/* => @app/view

Where is phpstorm getting @app from?
How can I extend this for the backend, frontend, console - advanced yii app?
How can I get this to work for packages in vendor folder?
What is "themes" ? I know the word, but that folder does not exist either in yii-app-basic?

nvlad / yii2support