Using Steam as a benchmark for Mac users will always result in lower percentages, as Apple has a more established gaming hub / store themselves.

For example, this game is on Apple Store (for Mac)⋮

https://apps.apple.com/us/app/resident-evil-village-for-mac/...

Not on Steam:

https://steamcommunity.com/app/1196590/discussions/0/3484123...

Same happens with Ubisoft titles.

But, regarding the source, here you go: https://en.wikipedia.org/wiki/Usage_share_of_operating_syste...

I do agree the way these are gathered is not perfect. But closer to reality, or would you assume Mac presence in lower than Linux?