Also known as Gary from Interlopers.net. I'm a retired Halo2X Modder and Source Mapper/Coder/Overall Modder. I'm a global moderator at both Halomods.com and Interlopers.net
Dynamic shadows from the sky. I call them Dynamic Sky Shadows.
I think it's the same method use in those videos show casing Cascaded Shadow Mapping.
This is the Alien Swarm engine. I basically just made the env_global_light entity create four ortho projectedtextures. Three of them project a texture with a hole in the middle, and one(the center) projects a fully white texture.