Tim
For more on this, see “The Moat Map”, which delves into distinguishing between these categories (aggregator vs platform) and the extremity of a competitor’s categorization (commoditized vs differentiated supply).

In addition, with respect to regulating aggregators, see also “The Principal-Agent Problem and the Chinese Wall Solution”.

Taken together, those two articles answer your subsequent questions, including:

[Tim Wu:] So iOS is a platform in Thompson’s language because you cannot publish iPhone apps without it. But then the App Store curates stuff and helps you find things you want using search. Thompson refers to the App Store as an aggregator. But does this mean that if you add a search function to any platform then it becomes an aggregator, too? If Bill Gates had, in the ’90s, thrown up an Application Store on Windows would he have been both an aggregator and a platform? What does that tell us about the antitrust action then? […]

Similarly, Thompson describes YouTube as an aggregator (level 2 or 3, I think) but it has obvious platform tendencies too.

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