I tried with Windows 7 (Firefox 115) and it reports Windows 7.
It seems though that it cannot distinguish between Windows 10 and Windows 11, so, without looking further, I suppose the detection is based on the User-Agent string? (The OS version browsers report on Windows is frozen, so Windows 10 and Windows 11 have the same version there.)
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It reports that my OS is Windows 10 on two different browsers, even though my OS is Windows 7.
Do you know what user agent the browsers send?
I tried with Windows 7 (Firefox 115) and it reports Windows 7.
It seems though that it cannot distinguish between Windows 10 and Windows 11, so, without looking further, I suppose the detection is based on the User-Agent string? (The OS version browsers report on Windows is frozen, so Windows 10 and Windows 11 have the same version there.)