In short, demographics are from the third party doubleclick cookie and are only available if you enable advertiser features.
Whether this enables Google to click a significant amount more of data on these third party cookies is unclear to me. So my original claim could be wrong. I would guess that a significant amount of the information would still be collected via AdWords tagging for conversions or display Ads.
There are also Apple and Android device IDs for mobile. I don't know if those are app specific or device specific and if they are only logged if advertiser features are enabled.
it seems pretty obvious visitors are absolutely associated with profiles of people.