>So McDonald's has a monopoly on the Big Mac?

yes, that's called a differentiated product in economics and it confers market power the same way any other monopoly does, Sony controls the Playstation ecosystem, Apple its ecosystem and it gives them leverage over the consumer, those products are not commodities.

That's why pharmaceuticals, patented medical devices or cable and broadband providers are regulated, see net neutrality legislation. Which exists exactly to commoditize information access in a market that's dominated by differentiated providers so they can't exercise their gatekeeping power to drive prices between you and online services up.