By default, the expression Yii::app()->user returns a CWebUser application component which represents the information that are closely related with the current user. Some information can be persistent throughout the current user session. For example, CWebUser already comes with a name property that stores the username of the current user.
The yii2-detail-view extension offers you an easy way to EDIT and VIEW your model data, toggle these modes, and adds other features. You can read the extension documentation and/or view a demo of this extension, to understand usage.
The customary configuration of a Yii application includes just a single database section in the protected/config/main.php file, but it's easy to extend this to support more than one, tying each Model to one of the databases.