How many other things are websites guessing? Are biased heuristics for the benefit of (a) the website user (the ad target) or (b) the so-called "tech" company

How many other things are websites guessing? Are biased heuristics for the benefit of (a) the website user (the ad target) or (b) the so-called "tech" company and its advertiser customers? What other HTTP headers are websites ignoring?