I think only the ball has to be over the goal line to score a touchdown. I saw a play in today’s Rose Bowl in which the player held the ball over the goal line where it meets the field boundary, and it was ruled a touchdown.
The goal line is extended indefinitely beyond the side line for the purposes of a score as long as the player in possession remains legally in bounds
The goal line is extended indefinitely beyond the side line for the purposes of a score as long as the player in possession remains legally in bounds