Why using both baking powder and sodium bicarbonate?
There are two reasons (that have also been discussed in many other questions)

Baking powder isn't just sodium bicarbonate + acid. It often also contains aluminum compounds that release gas when they are heated. That means they will make bubbles not just when the batter is mixed but also when it is baking.

Baking powder is ph neutral while baking soda is basic. Recipes will often include mostly baking powder with some soda to neutralize acidic ingredients.

See also:
What is the difference between baking soda and baking powder?