Hi Gavin,
Welcome to the Wix Code forums.
Not sure what you mean by “fully responsive”, but with Wix Code you can do just about anything? You want responsive? Well, with Wix Code, you can get responsive up the kazoo.
Start with the Wix Code documentation to learn what Wix Code is, what it can do, and then finally how to do it. Take a look at the Examples , load them into your editor, play with them, and see for yourself - responsive in action.
Have fun,
Yisrael