Is downloading Windows 10 free illegal?

Downloading a full version of Windows 10 for free from a third party source is absolutely illegal and we wouldn’t recommend it.

Does Windows 10 detect pirated games?

No, Windows 10 won’t “automatically check your version of the software and download software updates or configuration changes, including those that prevent you from accessing the Services, playing counterfeit games, or using unauthorized hardware peripheral devices”. It has no right to do so, based on the Windows EULA.