As the video explained, the net sum current flow for a properly working three phase system is zero. As each phase rises and falls, current flow is balanced between the legs.
On the positive line of the wave, current is flowing in one direction, on the negative (or lower side from the neutral line) the current is flowing in the opposite direction.
The ground fault detector wants to see this "0" total flow from the sum of all three phases.
A open or grounded phase would cause a net sum that is "not" 0. It doesn't care whether the system has a open or short..... it just knows that it is not seeing the zero total flow.
Alternating current and direct current cannot be visualized in the same manner. With DC, electron flow is in one direction only. AC has current flow in both directions depending on if the sine wave is above the neutral line or below it.
