Hello I am the developer of PlayTracker and I just got here and I see some confusion regarding our numbers on Insight so I'd like to clarify.
We get all our numbers through user sampling. The platforms we gather info from are Steam, Playstation, Origin, and XBOX. XBOX also has an endpoint for all Windows games and we use that to estimate total PC sales, when calibrated with our more accurate Steam database. For EGS exclusives that means EGS numbers. For Red Dead in particular we could only report total PC sales and Steam sales, as seen in our graphic, and quoted the videogamebiz tweet for the EGS numbers they got from SuperData. We gave no idea how SuperData gets their numbers, but figured videogamebiz was legit enough to quote.
I go into detail on our methods and the Windows data we recently made available via Insight here:
https://playtracker.net/insight/about/
I must emphasize that you need to be thorough when looking through our Windows platform figures as there are sometimes multiple entries for a single game.
For platforms that aren't Steam we offer the popularity score for free so you can get an idea of how a game did. Precise estimates, like those we offer publicly for Steam games, are available to supporters via Patreon. I know that kind of sucks if you're just an enthusiast and like data, but we still offer far more than others for free, and just 20% of what SteamSpy makes on Patreon every month would be enough for me to quit my job and work only on the site and that's something I'd really like.