• McDonald’s was just an example, the point is most apps don’t need to do that at all.

    I do happen to know how payment systems like that work, and thankfully those are all cloud-based, the only thing the app does is start transactions and check with the server if they’re paid. If they implemented it well, as I suspect a big corpo like McDonald’s probably would, their own order screen also checks server-side if orders are paid. Not much you can do from the app side to mess with that.