It is super easy to build responsive apps in weweb. Weweb follows the CSS principles so you start designing your app for desktop and then you gradually go down to smaller screen sizes by using the breakpoint buttons.
Any changes you make on the tablet breakpoints will be visible only on that size and below. Same for the mobile breakpoint.
The link you provided is from the developer documentation. This is the way to go when you are developping your own components.
All existing Weweb already implement this feature for the appropriate properties (width for example, or layout of the container). Slavo pointed you the way to use it for all existing components :).