Why do the first half and the second half of the World Cup semi-final cross?

Why did the upper half and the lower half cross in the semi-final of the World Cup?

1. The top four teams in the World Cup must be four teams to participate in the competition.

2. Four teams broke through the tight encirclement and were divided into two and a half areas.

There are two teams in the upper half and the lower half, and two teams belonging to the same half can't play any more. The two half-court teams play in a staggered way, and then decide the two teams that enter the finals.