I'd imagine they are responsible too, not just alone, and that Google is a much more attractive target for GDPR enforcement both because they're larger, have more money, are more visible, but also because they're directly facilitating the "different advertisers" sharing that info.

If Google ceases to provide them the means of readily sharing info then all of those entities will no longer be violating the GDPR, in the scenario anyways.

As I understand it, Google is responsible for not sharing information that would allow them to violate GDPR. Without explicit user opt-in, that is.