When I was taking cyber security, Sandboxing and Linux was one of the topics which was brought up.
Not sure when I associated it with the entire OS. It appears that the Host OS can be sandboxed for added security, and some containerized applications like Flatpaks are sandboxed. But not all applications are. Like the OS provided packages in most package managers.
Yes modern Windows is based on the NT Kernal. However to keep with compatibility with older programs, NT needs to be compatible with DOS. For most people they never saw the transition from DOS to NT, since it was quietly done with Win XP.