How is chewing food both chemical and physical?

It is both.

It is a physical change because when you chew your food you break it up into smaller pieces and are therefore changing its size and shape which are physical changes.

However, digestion begins in the mouth. When we chew our food, it mixes with enzymes from our saliva and begins to break down the food. Digestion is a chemical change.

You are still breaking it down physically, but you need your stomach acid to break down the food, chemically.