Confused

For each move, the robots take a pearl out of the box and put one of the opposite color back. The winner is the one who pulls the white pearl on the Nth step.

I'm confused. Does each robot take a pearl out of the box in each step? Or just one robot per step? The example picture rather clearly suggests that just one pearl is swapped in each step, which would mean just one robot per step. But "the one who pulls the white pearl on the Nth step" suggests that all robots get to do a swap each step. Also, if only one robot can swap in each step, then the winner can only be the one who is allowed to do the Nth step, and this makes no sense to me.