When you assemble a car, you generally don't put together the engine at the same time. You get the engine as completed unit, and put it where it goes (with a hoist or so) and bolt it down, etc. Sure, there's an assembly line somewhere else putting together engines, but electric vehicle assembly doesn't assemble the electric motor(s) during vehicle assembly either.

EV batteries take up a lot of space and that likely complicates assembly. My plugin hybrid has about 20 miles of EV only range, and loses 5 cubic feet of storage for the extra battery vs the hybrid model; 5 cubic feet is about 37.4 gallons.

I suspect battery costs are higher than you think.