Steam

That begs the question of whether or not people who refuse surveys are biased towards any particular OS. I'd guess they'd be more likely to use Linux than Windows, so the actual percentage is probably higher. But, unfortunately they are difficult to study.

Though not impossible. First ask if they prefer Windows or Linux. Then ask them if they are willing to participate in a study.

Doesn't mostly every survey have that pitfall?

Isn't it randomly batched out to the whatever like 18 million steam users every day and it automatically collects it?

I've never manually input my system specs into a steam hardware survey. Idk why you'd think people can lie or why using publicly collected data frequently and across a massive population makes someone an idiot.

Your way of interacting online seems unhealthy.