Earlier quoted context omitted.
Unity has a rendering pipeline coded for you, using DirectX or OpenGL, depending on platform and settings.
Isn't the point of this repo to compare the usability of different developer tools?
"This repository contains multiple implementations of the same 3D scene, using different APIs and frameworks on various platforms. The goal is to provide a comparison between multiple rendering methods. This is inherently biased due to the variety of algorithms used and available CPU/GPU configurations, but can hopefully still provide interesting insights on 3D rendering."
It's a mixed bag of APIs, software, hardware and tools, approaching the rendering in very different ways. There's even an offline renderer (Blender Cycles) and a software real time renderer in the repo.