There isn’t a “native” app it is just your web app wrapped into a nice app users can download on the app store. The wrapped apps have extra features like maybe sending notifications, getting GPS, etc. But they are really just your web app packaged in a nice browser.
So by developing your web app you are kind of updating your website and your app at the same time. It’s up to you to make the pages responsive from large widescreens all the way down to phone screen size.
Some people make them responsive across the whole range, but if the layouts need to be completely different then some people just make two different pages, one for desktop and one for mobile.