"The Premier League (and FIFA) uses Hawk-eye 3D imaging technology, which judges offside by the part of a player's body furthest forward (excluding arms). Several cameras are used to judge the offside line.
The Hawk-eye operative will select three frames for the VAR, who will choose the one that best represents that first point. From this frame, the 3D imaging is activated."
https://www.espn.com/soccer/english...?platform=amp#referrer=https://www.google.com
"In the case of offside, the RO will be able to generate a 3D line pinned to the attacker’s most advanced bodypart (arms don’t count) to give their VAR a definitive reading."
https://www.theguardian.com/football/2019/aug/09/var-premier-league-season
And from the Women's World Cup:
"Virtual offside lines are superimposed on the broadcast image by computer software. Angle of view, lens distortion, field curvature and many other factors are considered when calculating the true position of the these lines. The lines will be calibrated before each match by the technology provider to take into account the exact pitch dimensions and conditions on the day. The VAR team will have various tools available for determining offside positions, which have been validated in a number of tests across different venues by an independent third party using survey grade equipment."
https://football-technology.fifa.com/en/innovations/var-at-the-womens-world-cup/