The "dark" EV plants making cars aren't using any labour, let alone cheap labour.
There's also that thing that made China what it is today - the USofA using China for cheap labour and a place to outsource all the dirty parts of high consumption.
They are most certainly using cheap labor to design, plan, operate and maintain those "dark" plants.
Very highly automated production lines (cars) have similarly moved along the labor-price gradient in Europe (~east) during the last decades, despite language barriers: You need some engineers, electricians, constructions workers, janitors, etc regardless, and those still scale with general labor costs, even if there's no human hands in the assembly line.
The US doesn't have those because we're not even trying.
China is capable of it due to the absolutely brutal grinding work culture. They're not superhuman, it's not Tony Stark building the Iron Man suit in an evening, they're smart and work extremely hard.