The Photorealistic Render plugin is responsible for handling the PBS (Physically-Based Shading/Rendering). You can find more information about PBS/PBR here. The plugin uses the Ogre v2.1 engine, which is an Open Source Grafics Engine. You can find the source code of the plugin at ..\ApertusVR\plugins\ogre21Render.
With this plugin you can easily add any objects to your scene with PBS/PBR materials. These materials are called HLMS materials in Ogre. The basic format is Ogre's .mesh, it is usually have a .material file and even pictures. To build your own object that uses PBS materials you can use the HLMS Editor that uses Ogre. With the correct settings you can load glTF 2.0 files too.