At some point, biters almost always overwhelm my defenses

you need to have a wall of gun turrets fed by a ammo belt, all around the base. They will shred through all biters.

How do you start this? expedition terminal says there are no current expeditions

But then who is putting everything on the treadmills on the other side?

he has a yellow inserter that will pick up food from the fridge and insert onto the belt