Asides from the referee, there are other officials in the game.
The umpire is responsible for confirming the equipment of the players before the game starts. They also watch the game, especially the dividing line between the offensive and defensive players. The umpires make sure that each team does not have more than 11 players on the field. They are responsible for calling most of the holding penalties. The umpire assists the referees when it comes to taking certain decisions, especially along the line of scrimmage.
This is also another vital official, who is responsible for watching encroachment, offsides, and other scrimmage violations. They monitor the chain crew and grabs when they are measuring for a first down. They assist the line judge for illegal motion calls and other unlawful movements on the pitch.
The line judge is at the opposite side of the field, away from the head linesman, and their role is to help in monitoring illegal shifts or movements. They make offside and encroachment calls. They also assist umpires in looking out for any unauthorised use of hands on the end of the line. They supervise substitutions made by teams, and they also oversee the timing of the game.