Where is the end zone plane?

Answer 1

The imaginary "plane" which separates the end zone from the rest of the playing field is right at the very front of the goal line, and extends upward and outward to infinity.

Therefore, if any part of the ball is above any part of the white goal line (thus "breaking the plane"), the result is a touchdown. This includes a situation where the ball breaks the plane over the sideline, as long as the player possessing the ball is still in bounds.