The first one seems most likely to be the relevant answer to how this was tracked: https://adwords.googleblog.com/2017/05/powering-ads-and-analytics-innovation

The first one seems most likely to be the relevant answer to how this was tracked: https://adwords.googleblog.com/2017/05/powering-ads-and-analytics-innovations.html https://adwords.googleblog.com/2017/05/powering-ads-and-anal... > Google’s third-party partnerships ... capture approximately 70% of credit and debit card transactions in the United States See also https://www.bloomberg.com/news/articles/2018-08-30/google-and-mastercard-cut-a-secret-ad-deal-to-track-retail-sales https://www.bloomberg.com/news/articles/2018-08-30/google-an... and https://www.technologyreview.com/2017/05/25/242717/google-now-tracks-your-credit-card-purchases-and-connects-them-to-its-online-profile-of-you/ https://www.technologyreview.com/2017/05/25/242717/google-no... - Google has deals with the credit card providers to get purchase data so they can link them back to your online behavior.