> Why do they need to know my screen brightness, memory amount, current volume and if I'm wearing headphones? This is clearly adding entropy to de-anonymize users between apps, rather than to add specificity to ad bids.
Everything listed changes way too often to be useful for tracking. My guess is that it's for anti-fraud purposes. Someone setting up fake devices and/or device farms is likely to get similar values, which means they can be detected via ML or whatever.
None of those are likely to change when you navigate from one website to another, with tracking/ads disabled, which is what they want to be able to track. Otherwise they'd just use their cookies.
One device visits a site where you sell ads. A minute later, an unknown device with identical battery, volume, headphone, brightness, model number, browser version, and boot time to the second arrives on another site you run ads on. There's a pretty good chance they're related, because the odds of all those being the same plus those two sites and recent timings involved is rather low: https://coveryourtracks.eff.org/
Plus it doesn't have to be perfect. It just has to be good enough in bulk to sell.