tvOS 12.0.1 can be downloaded over the air through the Settings app on the Apple TV by going to System -> Software Update. Apple TV owners who have automatic software updates turned on will be upgraded to tvOS 12.0.1 automatically.
It is not clear what features or bug fixes are included in tvOS 12.0.1, but given that it's a minor 12.x.x update, it has likely been released to address issues discovered in the tvOS 12 update.
If we discover what's new in tvOS 12.0.1, we'll update this post with info, but Apple often provides little detail on tvOS updates.
tvOS 12.0.1 follows tvOS 12, an update that introduced support for higher-quality Dolby Atmos sound, zero sign-on, and new aerial screensavers created in collaboration with the International Space Station.