At exactly 90° South latitude, the sun rotates in a near-perfect counterclockwise circle around the sky. There is no difference whatsoever between “night” and “day”. On any given day, 2pm looks the same as 2am. The sun is at the same height, just in a different part of the sky.
The sun reaches its highest point in the sky, about 23.5° above the horizon, around the December Solstice (December 21).
After the sun reaches its peak, it begins to set. It’s still rotating in perfect circles around the sky; they’re just getting lower and lower each day.