Most use one giant vat (or something to that extent) to fill up the tub, then they use a big suction-draining device (or something to that extent) to empty the tub (back into the vat), and usually do not change the water for a very long time. Most tubs don't get completely, 100% drained and water is left in parts of the plumbing.
However, the amount of water left in the tub generally will be diluted by the new fresh water, and whatever you use to sanitize it will take care of any potential issues.
This is my experience with carrying a few different manufacturers and seeing their production, as well as dealing with the customers after their purchases.
However, I would like to hear if anyone else can chime in as to hear their experiences...