RE: Transparent 3D models
As Ken mentioned, you have to convert your models into gITF (.glb) format. However, I personally haven't had much success in achieving transparency with holograms in the HoloLens app. One example is the automotive headlamp assembly with its clear polycarbonate lens. Even though the lens may look OK in the Guides previewer (transparent lens), it comes out opaque when placing the hologram in the HL2 world.
My workaround has been to separate the transparent part (lens) from the main mesh (headlamp assembly), and place both parts in the real world as individual holograms. I then apply the X-ray style to the opaque part (lens) to make it transparent.
The result isn't optimal (the lens ends up with a disagreeable bluish glow), but at least the user can see through it.
After my failed attempts, I stumbled upon the note below in the Dynamics 365 guides documentation (wish I had read this sooner):