They don't need to engineer them, but for example BYD's vertical integration with batteries has been their superpower. Tesla once upon a time (~circa 2013) talked about building battery manufacturing "gigafactories" all over America but I don't know what the status of that is.
That is likely increasingly less relevant as battery technology matures. Back in 2013 the situation was entirely different, and off the shelf cells were all designed for things like laptop computers.
