Virtually all google products are governed by a shared google privacy policy, which allows Google to combine data across products and services. This change was made years ago, despite some end user protest.
Most apps and products are also optimized to maximize data harvesting - ex. Android OS and Maps will nag if users don't give up data. So I don't really believe what you're saying.
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.
No, they literally could not, because the data flow between Gmail and Ads has ben severed a while ago:
https://blog.google/products/gmail/g-suite-gains-traction-in...
These kind of things are treated pretty seriously. There are teams whose job is literally making sure that your data does not leak from one Google product to another in a way that would violate your privacy. Source: I'm an engineer in Google, but in none of the mentioned teams.