It is adjusted according to the batter. It’s from roughly the knee to just above the letters on the shirt. For a shorter batter, it’s a shorter zone.
I am inclined to overthink this, but it appears the vertical zone is in play according to batter, but the horizontal is not. The width of home plate, which never changes, will determine (for lack of better words) the east-west zone. I would be interested in a 3D depiction of a strike zone, and find it a bit surprising that it would change from one batter to the next, but that makes sense now that i overthink about it..