The dome created an ecosystem of it's own, on part of alien terraforming techology.
The aliens are accustomed to zero-G environments, and are soft and fragile, so they developed metal skeletons to act as bodies and weapons at the same time. Here's an infested human.
The Alien weapons could be torn off from the defeated aliens and wielded by the player, thanks to an alien infection/interface in the nanosuit he wore. These are a sniper rifle (Cut through to show internal structure) and a gun with alternate fire modes. Gregor Kopka did the initial design on this.