frozenpeas I think you might be overlooking some of the basic structure you are asking about.
If you build a website that accesses data from MySQL, FileMaker, or any other database system, then inherently you should be able to make an app in C2 or any other programming language that accesses the same data.
If you are more wondering about visuals of the website/app, that's a different story. But honestly I would suggest against that anyways because generally you would design the usability and layout of an app a bit different from how you would set up your website.
So I could be way off from what you are asking but my suggestion is that if you get a good database set up that can be accessed via the internet, most programs should allow you to get your data. As an example, my boss owns a liquor company (Rumchata) which has a website and a backend database that has recipes for the liquor to show on the web site. I then made an app through C2 for Android and iOS that accesses that database of recipes through AJAX and can show all the same recipe info that the website has.
Sorry I rambled a bit, but hopefully that helps a little?