"The potassium sorbate ends the fermentation process when you have reached the gravity you are looking for."
It does not end fermentation. it prevents renewed fermentation when you back sweeten.
Potassium metabisulfite, or Sodium metabisulfite (K-Meta) is a common wine or must additive, in which it forms sulfur dioxide gas (SO2). This both prevents most wild microorganisms from growing, and it acts as a potent antioxidant, protecting both the color and delicate flavors of wine. it does not stop fermentation either.
there are only 2 ways that fermentation stops. 1 the alcohol content is so high the yeast can not survive. or it has fermented to dry (less than 1.00sg)
It can get stalled but that is another subject