The difference between is acid
Baking soda has only one ingredient and is sodium bicarbonate.
Baking powder has more acids like monocalcium phosphate and one may be sodium acid or sodium bicarbonate and react differently . Is more as a delay reaction or also called double acting . Baking powder will react then at any moisture releasing Co2
In order for your baking soda to be added in cakes or breads it needs to be activated by an acid. Think of sour cream cakes products , the CO2 will react instantly only in a presence of acid