For those billions, Apple links to the Google search results page in Safari when you type a query in the iOS system search field. That page includes Google search ads. According to the 2017 annual report, Google made $77 billion in revenue from “Google properties revenues”, which is mainly search ads but also includes other sources such as YouTube. Ads served by Google elsewhere (such as AdMob or DoubleClick) aka “Google Network Members’ properties” are only $17 billion. Search intent is an extremely strong signal for ads even with no personal information, although the ads are probably personalized if the user is logged into Google in Safari.

$9 billion is the “traffic acquisition cost to distribution partners” in 2017, which includes payments to Apple as well as other entities that send people to Google properties. This cost comes out to only 11.6% of the associated revenue. It’s likely that the amount of money paid to Apple is contractually linked to the revenue Google gets from ad clicks that can be tracked back to a link from Apple.

So there’s really no need for theories about the abstract value of user data here, it’s a simple referral fee for the ad clicks that are Google’s main business.

For actually serving Siri search results directly, I'm not sure what benefit the search provider gets. My guess is they're really after the "Search Web" fallback if you're not satisfied by Siri's initial synopsis, which will take you to the search page (and therefore the search provider will now know who you are).