When users search for “windows 12 iso download 64bit 2021,” they are likely confusing the release of Windows 11 (which happened in 2021) with a hypothetical future OS. Windows 11’s build number is 22000.xxx. There is no official build number for Windows 12.
Microsoft tests future features via the Canary Channel (formerly the “Windows 12 test ground”). To enroll: windows 12 iso download 64bit 2021