... What I do know is that the layer has to be thick in order for it to work for both PC and Xbox. They are basically treating Xbox as PC now.
They've been treating Xbox as PC since the OG Xbox. Xbox is a contraction of Direct X Box - because it was a standalone piece of PC-style hardware powered by Microsoft's DirectX API and a derivative of the NT Kernel.
For the Xbox Series X, Direct X 12 Ultimate is employed. One of the big shifts for DX12 in general, but U specifically, is optional low-level access to the hardware, and a focus on pure efficiency. Funnily enough, this was done in response to Mantle and other APIs which were offering developers much greater flexibility in the PC space, though restricted to AMD's cards in Mantle's case. Mantle, Vulkan - these appear to have lit a fire under Microsoft's ass. For the Xbone, DX11.x was used, which was nice but bloated, and didn't offer the same optional low-level access as DX12 - its more in line with Sony's GNMX, the higher-level access layer on PS4. Comparisons for DX12 have been made to GNM, which is the low-level access layer on the PS4, but with the added benefit that DX12 is cross-vendor compatible: it works on any hardware that supports DX12, meaning optimisations for the XSX|S platforms can be carried over to the PC platform, and vice versa.