There are lots of location that mechanical digestion occurs. But 2 common locations of mechanical breakdown of food (or physical digestion) occur in the mouth and in the duodenum.
Mechanical breakdown of food does not require enzymes, which is different from chemical digestion. Usually, the main function of mechanical digestion is to help speed up the chemical digestion later, usually by increasing the surface area of the food.
In the mouth, we chew the food. This can help increase the amount of surface area of the food we ate so that later on, the chemical digestion that requires enzymes such as amylase or protease will speed up. More enzymes can contact the food.
In duodenum, bile that are produced from the liver helps emulsify fats and lipids into small oil droplets. Again, this action can also help increase the surface area of the lipids so that the digestion of lipids with enzyme lipase would be more efficient. Note that bile is important because only enzyme lipase can digest lipids in the duodenum, so if bile is not produced, fats are hardly digested.
Other types of mechanical digestion may include the churning in stomach, or the squeezing of food in the oesophagus (peristalsis).