https://wccftech.com/download-tvos-12-gm/

Developers and public beta testers can download tvOS 12 GM for Apple TV 4 and Apple TV 4K right away.

Probably the biggest highlighting feature of tvOS 12 is the added support for Dolby Atmos. Of course, that is only possible if you have an Apple TV 4K along with supported audio equipment and source of content.