After thinking about this some more I think you are right about apps running in browsers. Looks like the IG web platform is written in JavaScript using a well-known framework.
Makes sense to think of it was an app. 😊
I would think there wouldn't be much difference in terms of the messages which are sent between the app in the browser and a MT4 client?
e.g. both would communicate with the broker's server such as sending request messages for price bar info to display and sending execute messages for buy/sell orders.