Did some research in some .xbg files, DJ Normality posted them here:
https://drive.google.com/file/d/1IVS3Ef ... rwXmy/view
Verts appear to be split in some or most files if not all, example file: CABBAB432E7A1932, verts appear to be short signed.
In said file: 0x1130 short sign, 10209, padding 2
In same file: 0x044180 - 5478 - short sign, 2 padding
Probably nothing new for more experienced people like bigchillghost or shakotay or daemon or akderebur so have to wait for someone more experienced like them to figure it out. Faces I could not figure out unfortunately, pretty sure they're byte but couldn't get any of the files to work, verts did however work with short sign but like I said they appear to be split throughout the file.
EDIT: After looking at the file once more I realized at the top there's LOD0 - LOD1 - LOD2 listed, so the split verts etc are most likely just different LODs packed into the same file for performance.
EDIT 2: (thanks DJ Normality)
If you look at file A08F7270CE7E2467 - here you go, faces appear to be short, verts also are XZY format, also this is probably LOD1-2, couldn't get faces for LOD0
If you also change the vert offset to 0x8b00 it's the exact same verts count but faces don't match, possibly because different LOD?, unsure what this means.
Now when it comes to bones, weights etc I can't help since I don't have that knowledge or when it comes to automating etc. Materials appear to be at the very top/start of the file too.
EDIT 3: Also found multiple split parts, if you open the biggest file in the link above it's a helicopter and I was able to find that the interior and exterior were separate so keep this in mind also.