Goal-Line technology is used to determine if the ball has completely crossed the goal line or not. The International Football Association Board (IFAB) has requested Hawk-Eye, which is the company that Goal-Line technology belong to, not to interfere with the game, and so, Hawk-Eye has given a special type of watch to the main referees so they can receive a signal determining whether a goal was scored. This signal is transmitted to the watch in less than one second, so it doesn't interfere with the game. This signal will only be transmitted to the match officials, unless the organiser of this match or tournament wants show a replay, but this information will still only be available for the referees of the match.
Magnetic fields is one way to determine when a goal is scored. Before the match starts, the technicians have to install cables under and around the goal. Inside the football there will be receptors, which will allow magnetic field to detect if the ball crossed through the goal line. When the receptor of the ball interact with the magnetic field, the operating system will calculate the precise location of the football and determine if a goal was scored.
Camera based is another way to determine a goal. There are 7 cameras per goal, that are set inside the football stadium. These cameras detect the position of the ball, and then the video footage will be analyzed by the operating system to see if the ball has crossed the goal line.