If you are coming over to Yii 2 from Yii 1.x, you may have already read this useful wiki for creating dependent dropdowns.
You can use a similar approach in Yii 2 to do the same. But if you are looking at a prebuilt solution that helps you manage it easier, read along.
To extend the functionality of you web application relative to your requirements you are supposed to use existing yii core libraries or use external libraries. There are some steps to ensure security, uniqueness, modularity, performance and to avoid rework in future.
Sometimes the active form we wish to use to edit/add a new element on our database is too small and we believe that is much better to use an AJAX'ed dialog/slide form rather than reloading the page to just display one or two fields.