I like the grocery store analogy. You can get Kellogs Corn Flakes at any grocery store. You can get General Mills Cheerios at any grocery store. You don't have to make separate trips to the Kellogs store and the General Mills store. There are exclusive store-brand items, for which you do need to go to a specific store, but there's minimal additional cost to do so. You don't need to buy a new car to get from Trader Joes to Wegman's, you can use the same car to go to both.

With app stores the situation is reversed. Exclusives are common, and many quite expensive devices only have one store available. And once you've got an expensive product from the Apple App store on your Apple device, you can't (usually) change to a Google device and the Google Play Store and still use the product without re-buying it.