A market is the set of things which are fungible substitutes for each other. Essentially every brand of gasoline is in the same market. By contrast, gasoline and diesel are separate markets, because some vehicles require gasoline and can't use diesel and others the other way around. They're not substitutes when your vehicle can only use one of them and you can't get out of it by trying to claim that the market is all types of fuel.

Now suppose you have an iPhone and want to download an app, or want to distribute your app to customers many of which have an iPhone. What substitute for Apple's store is available to do that?

We aren't talking about nerd definitions. We are talking about the real world.

Apple can change their rules and it materially impacts the market.