I believe usage for ads is highlighted on the non-legalese privacy page because it's a common question/concern, but the terms specify that data can only be processed as requested by the customer (IANAL, but that's my read and understanding as a Googler who works on Cloud infrastructure).
I would go further and say you're crazy if you think a Google engineer is going to run code on your data without a requirement in a signed and funded agreement.
Can you provide a source, either in the form of a ToS/privacy policy or an evidence-backed article, showing that Google uses customer data stored in GCP for advertising purposes?
Google doesn't use data stored in GCP for their own purposes any more than Amazon uses data in AWS or Microsoft uses data in Azure.
That said, it's completely understandable that companies don't want to help fund competitors. --Walmart knows that they could use AWS without having any real fear of Amazon employees looking at their data to get a competitive advantage, but they chose Azure because Microsoft is not a direct competitor.
Likewise, companies providing healthcare software look at what Google is doing in that space and they don't want to fund the competition by using GCP.
These are rational decisions that we would expect companies to make. It's actually more surprising that, for example, Netflix still uses AWS given the fact that Amazon is a direct competitor in the streaming business. --GCP or Azure would make more sense here unless Amazon is giving them a really good deal to keep their business.
What do you mean by this? All major corporations use private internal data to drive their products, including competing with their own customers. Amazon's ecommerce business is notorious for this and a reason why other major retailers tell their vendors not to use AWS. This was all just discussed days ago: https://news.ycombinator.com/item?id=22079174
That being said, and with 15 years of adtech of experience, I guarantee that there's absolutely no usage of business customers' first party data by Google's adtech system. While companies can use it to match and target their users, they have to do explicitly with lots of restrictions and protocols and it's only become more limited with new privacy regulations.
Which doesn't mean they definitely won't do it. Businesses do illegal things all the time for competitive advantage. Literally the entire advertising industry, including Google, is flagrantly violating GDPR.
