Unity is a cross-platform game engine developed by Unity Technologies.
It supports multiple platforms such as desktop, mobile, console, virtual and augmented reality platforms.
The engine can be used to create three-dimensional and two-dimensional games, as well as interactive simulations and other experiences (IA, AR/VR, film, engineering …).
The UI allows to easily add and move elements in the game world, while scripts are coded in C#.
The proof of concepts can be built with the personal, free license.
If the project is carried further, some license may be required (strongly depends on the company income). Please see here
Getting started with Unity#
The scales in Unity are unit-less. For the Hololens and mixed reality in general, 1 unit corresponds to 1 meter in real life.