Apple has released the first beta versions of iOS 12.1.2 and macOS 10.14.3 for users participating in the system software beta testing programs. The initial beta builds are currently available to those in the registered developer beta testing programs, but typically the public beta rolls out soon after.
Additionally, Apple has rolled out the first beta builds of watchOS 5.1.3 beta 1 and tvOS 12.1.2 beta 1.
iOS 12.1.2 beta 1 focuses on bug fixes and improvements, and it’s probable that no new features or major changes are expected.
Those who are enrolled in the iOS beta testing program can find iOS 12.1.2 beta 1 available now from the Software Update mechanism within Settings app.
MacOS Mojave 10.14.3 beta 1 also likely focuses on bug fixes and improvements for the Mac operating system, and it’s unlikely that any major changes or new features would be included.
For Mac users beta testing macOS Mojave, the MacOS 10.14.3 beta 1 release can be found in the Software Update section of System Preferences.
Apple usually goes through a series of beta builds that are tested by developers and public beta users before issuing a final stable version to the general public.
The most recently available stable builds of the various Apple operating systems are currently iOS 12.1.1 for iPhone and iPad, MacOS Mojave 10.14.2 for Mac, and watchOS 5.1.2 for Apple Watch.