There's a lot of different stuff out there, actually, of 3D stuff Sega played with on Saturn. There were a few different ideas they were juggling. But instead, we ended up with Sonic R. I don't know what they were thinking with that one.
But that first video; I'm questioning that it's actual Saturn gameplay footage. The reason being is that the transparency effect actually works really well when that wall goes out of view so you can see Sonic inside the mountain-side. The Saturn was TERRIBLE at transparency effects.