The Asus Zenfone 6 runs Android 4.3 with the ZenUI, as seen in the other phones in the Zenfone range. This isn’t the latest version of Android, but differences between core versions of the system software are often steamrollered by changes made in the custom interfaces that sit on top anyway.
The ZenFone 6 you're thinking of is the 2014 model. This forum is for the 2019 version. Don't expect the performance on that phone to be good, It's a 6 year old phone with a 2 core processor and it's a 32nm