how do they make professional teeth whitening trays in the lab?

I am curious as to how they turn a bite imprint into a little plastic mold for your teeth-like for bleaching or for invisalign. What kind of certification is required? Is this done by a dental lab tech, dentist , or what?
--------------------
After the impression is made, lab stone is poured into the impression to create a stone model of your teeth. Once this is hardened and trimmed, it is placed in a "suck down" machine that heats up a sheet of plastic that is dropped over the model and a vacuum is used to form it around the teeth. Then is it trimmed to just cover the teeth. If a dentist has the proper equipment, he/she can do it in their office, or train an assistant to do it. If not, they can send it to a dental lab. No certification is needed to fabricate it, but there is legislation that requires impressions only to be done by a liscensed practicioner. I'm not as familiar with the invisalign technique, but it does require someone with more experience in orthodontics because the teeth need to be slightly shaped with wax or trimmed in order to put the pressure on them to move them. Sometimes with bleaching trays "resovoirs" are placed on the front of the teeth by adding composite to make a spot for the bleach.
Source