I think they mostly use the internet. The browser shows your OS to websites via User Agent. Tracking downloads wouldn’t be accurate at all for Linux users because we’ve all tried several versions in the same time frame that a Windows user sticks to one version.
Use Agent can also be spoofed. There are browser extensions/addons to do it. Statistics services buy the info from large websites and of course google.
“I think they mostly use the internet. The browser shows your OS to websites via User Agent.”
That would let the cat out of the bag. How would these websites be able to accurately count individual unique devices aside from just general OS type? OS yes, number of unique devices for sure? They are not supposed to have that intrusive capability right?
But even with webserver stats it could not be very accurate. Not everyone goes to the same sites... And not all sites contribute that data or participate in the survey. If they are getting numbers from Google they are not going to be accurate for Linux either. Most Linux users are intelligent enough to avoid Google and use script blockers that block Goggle services on sites.