Yeah looks like it's a bit of both. The system generates the lines and the official makes a decision.

https://football-technology.fifa.com/en/innovations/var-at-the-world-cup/

THE VIRTUAL OFFSIDE LINE

Virtual offside lines are computer-generated lines projected onto the broadcast image of the field of play to help the VAR determine whether an offside offense has occurred. The offside lines used are the best possible and most accurate lines that can be generated with existing technology due to calibration using multiple synchronised camera angles.

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.