from the PL webby
the red line is so far off the shoulder or armpit...
https://www.premierleague.com/news/1488423Crosshair
These are two lines that are positioned for the defender and attacker.
The offside lines are drawn against the parts of the body of attacking and defending players that can be used to score goals. (These parts of the body are the same for all players, no matter their playing position)
The lines can also take into account parts of the body off the ground, such as a player’s shoulder, which is shown as a 3D vertical line.
The positioning of the crosshair is manual, with a line one-pixel wide, so that the exact position of the offside line and the relevant body part can be accurately identified by the VAR.