As an example, Facebook might sell you the ability to target people between ages 12 and 20 located in major metropolitan centers in the US. You never bought any user data that let you flag individual users, but you are still "using" the data.
(Disclaimer and also claim to authority: I worked in ad tech.)
It's the same with Google, you cannot control your data and what they do with it.
I'm not sure why people keep saying that Google and Facebook is selling user data. Does that mean that if those companies started to actually do it, it wouldn't bother the claimants as they consider it being done already? Seems to me like an important distinction to make.
In comparison, are Uber selling software? Rather than using software to sell a service?
Can I find out what data Google has on me?
Amazon doesn't directly sell you the AWS infrastructure. But they rent it to you. Google is doing something akin to that. If they had an opt out saying "Don't use my private data to show me ads" I'd no longer make the point they sell my data. But they don't provide any opt-out, and the effect is identical to my data being sold to advertisers.
If a company sold your information, you can say bye bye to being able to restrict what that information is and who has it; you might as well consider it public from that point on. I don't think anybody in their right mind would call that difference a technicality.
Google does provide the opt out you say they don't, too: Browsing in incognito mode.