The idea behind this is actually You have to create the Web Application first. And make sure it is fully responsive so that if anyone visits your website or web application with a different type of device, it should work properly.
Once your website or web application ready, time to make a mobile app that will load your website or web application inside it. Incase of Android, you can use WebView to load the webpage inside it. 
Another idea is that you can use rest API to load web data into your mobile app.