The jimu.js file does not only support a responsive UI layout but also provides functions for developers to develop responsive Web AppBuilder for ArcGIS widgets.
There are two ways to achieve the responsive goal, CSS and layout dijit. The CSS classes can be found in jimu.js/css/jimu.css.
One row of the layout.
The width is half of its parent.
The width is one third of its parent.
The width is two thirds of its parent.
The width is one fourth of its parent.
The width is three fourths of its parent.
The layout dijit can be found in jimu.js/dijit.
If both of these cannot meet your requirements, follow the best practices for the responsive UI .