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This is the correct answer. Caches in local PoPs https://cloud.google.com/cdn/docs/locations

They even have a map.

https://peering.google.com/#/infrastructure#edge-nodes

It also says "Static content that's very popular with the local host's user base, including YouTube and Google Play, is temporarily cached on edge nodes. Google's traffic management systems direct user requests to an edge node that provides the best experience."

Although note the map is only city addresses. For example the "London" pin is on the notional point where "London" is, ie Charing Cross. There is no giant network interchange at Charing Cross, it's just a convenient place co-ordinate for "London".

There is a big difference between 'caches in ISP networks' and 'caches in public datacenters adjacent to local exchange points'. The first implies preferential treatment from ISPs, the second is commodity available to everybody. Your link implies these local PoPs are more likely the second case.

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