Anyone knows what kind of BVH is used in Unity's new Physics?

I am trying to implement some Bounding Volume Hierarchy(BVH) in parallel using Job System for my own intersection/raycasting system, and searching ways to do that, I found Unity already has it in the Physics package! so I thought why not learn from it. But the problem is I don't know what kind of BVH is actually used.

I tried to understand how it is built by reading codes in BoundingVolumeHierarchy.cs & BoundingVolumeHierarchyBuilder.cs but I couldn't get it. There are only a few comments...

Anyone knows what kind of BVH is used in Unity's new Physics?