This article discusses software internally used by Apple.
Acquiring a copy without Apple's consent is illegal and may result in being scammed.
This build is most likely based on an internal build of iOS 12.0. It was first found on a iPhone XR prototype but may support other devices. Strangely, the app icon layout in SwitchBoard.applooks simalar to older firmwares such as HoodooYabuli 9B3176n