Baking soda is a chemical substance, the scientific name is sodium bicarbonate. When heated, it is easy to decompose water, carbon dioxide and sodium carbonate. In the process of food production, it is mainly used as a leavening agent, and carbon dioxide exists in food in the form of bubbles.

Yeast is a dry microorganism, and there are yeast species. When the temperature is suitable, yeast can multiply in large quantities, and in the process of reproduction, it can ferment and decompose food to obtain the energy and nutrients it needs. It also produces gas.

The combination of baking soda and yeast makes food ferment and swell. Reference.