Goal line: computer vision that tracks the ball
Deciding whether a ball crossed the line is hard: it happens very fast and there are often players blocking the view. Goal-line systems solve this in two ways.
Some use a magnetic field and a sensor in the ball. Others use several high-speed cameras around the goal and computer vision, a type of artificial intelligence that locates the ball in each image and, combining the cameras, calculates its exact position in three dimensions.
If the ball fully crossed the line, the system sends a signal to the referee's watch in an instant. The decision is his, but now he has an objective confirmation.
These systems need to be well calibrated before every match, and can struggle when several players block the ball from all cameras. That is why they are tested and certified, and why they only answer one specific question: goal or no goal.