If you're using it as an NFC payment method, then Google, like any credit card processor, only knows the merchant category and id (and the total purchase amount). They do not get a list of individual items.
Of course, if you buy from a liquor or grocery store, or a flower or gun shop, then those are big hints, although the details are missing
Apple is a master of words. It helps to read all of Apple's security documents.
With in-app payments, Apple collects app, merchant, amount, time info.
With NFC purchases, Apple collects location, and time of purchase. (I think they could use this one day to provide a map of Apple Pay compatible merchants. It also helps them corroborate their expected bank royalties.)
In addition, their contract with the banks requires the latter to give back "nearly three-dozen categories of quantifiable information," according to those who've seen the documents.
So when Apple says they don't collect detailed info during an NFC purchase, they're telling a truth, but not the whole truth. They're not collecting actual purchase info themselves, but they're still getting tons of useful purchase information fed back to them.
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Personally, I think that a proxy buyer (like Google Wallet used to be) is far more useful as a privacy tool, than the way that Apple Pay allows the merchant category & id - i.e. store name and location - to flow to the banks.
Think about this: let's say you buy a bottle of champagne and some flowers during a work day, and rent a little hotel room out of town. Meanwhile, your wife buys something at MacDonald's using her credit card back near home.
With a proxy like the original Google Wallet in front, the credit card companies only see a generic purchase. At worst, you might view more ads for flowers and hotels for a while because Google intercepted that info.
With Apple Pay passing everything through, the impact on you could be much worse. The CC providers know where you bought, and the categories. This flags them that you're probably fooling around during work hours and thus could be a future credit risk. Boom, your credit limit gets lowered, or at the least, you're put on a watch list.
(All CC providers have algorithms for this kind of thing, as part of their fraudulent purchase flag methods, and also to keep an eye on our future credit risk. They watch for behavior like suddenly attending therapists, purchasing a lot of liquor during our normal work hours, buying something at a job fair, or anything that might indicate we've lost or are losing our job, or might have a major family problem coming up.)
The upshot is: you want privacy? Use cash
