Hello, it is not like Rust/survival. It is similar to Team Fortress 2 like game. So you can play as human or as alien in designed maps. Humans are able to kill other humans but they are your teammates so you should not to do that 
You should kill humans only when you are alien.
And about that base building - you will start on map with default base and you can make it better or you can choose better place for your base and place there turrets, healing pods, armoury… and most important - spawns. When you die then you can respawn on these spawns. If your team will lose all spawns and there will be no builder and no armoury (there you can buy weapon which can be used to build other structures) then rest of humans will probably die soon (no ammo and no respawn) and aliens will win.
We will release video from game soon so it will be better to understand how it works and looks.