New
#11
Since I don't see an explicit answer to both parts of your question, here is my attempt to do so:
Windows 11 is a new version of Windows which tries to enforce some hardware security features which are entirely optional in Windows 10. It also implements some changes to the user interface.
Windows 10 21H2 is a minor update to Windows 10. It is not Windows 11 21H2.
The 21H2 suffix just indicates when their updates were released. Since both versions of Windows were released in the second half of 2021, both versions have the same suffix.