I can't even tell what you mean by "data harvesting" in general. But the fundamental error you're making here is considering "Google" as a single entity. Frankly, with that kind of thinking you can't win, because either Google, Apple or Microsoft has
all your data. No need of harvesting, you have literally gave it all to them and asked to keep it for you.
However, once you allow the idea that there are powerful people inside these companies, that genuinely don't want the company to create a panopticon, things can get interesting. I can't exactly prove that to you, but you've seen hints of that in the news over 2018. Or, you might have noticed e.g. that safe browsing only sends hashes of the URLs you type. Or, now this Gmail not telling Ads which languages you speak thing.
Now, again I can't offer more than my word for it, but the Gmail-Ads situation is the rule, not the exception. The only reason this made it to the news, is that the exception in place before has been lifted. But in Google, by default, if a product wants data you shared with another product, it has to make a very strong case that that's covered by intent of a permission you already gave. That, or ask you for permission, or ask you for the data again. If you ever wondered why Google keeps asking for the same basic immutable data, that might be the reason.
Funnily enough, this whole privacy thing is hard to tell from the outside. As you said, there is a single privacy policy, that governs the relationship between Google and yourself. All the internal data siloing is, well, internal.
Edit: I won't be continuing this thread. The cognitive load to not leak anything non-public is a bit unpleasant.