I watched a video about it a while back. The computer tracks something like the past 1,000 swings for each player. It records the zone at the moment the bat crosses the plate, and takes the average zone from those swings. They did it that way to eliminate the zone being affected by a huge crouch preswing or something like that.
They talked about players just called up with no data, but don’t remember exactly what the plan was. I think it might record during BP or something like that to get the initial data.