As a single market, US is probably biggest. I’m seeing numbers that say that the “Americas” is a bit less than half of global revenue, and that would include Canada and all of South and Latin America. So the rest of the world is of course very important to Apple, at least financially.

> doesn't mind using WhatsApp for messaging

Well WhatsApp was super early and way ahead of any competition, and the countries where it penetrated had no reason to leave, so it’s not exactly like they settle for less. It has been a consistently great service (in the category of proprietary messaging apps), even after Zuck took over.