You cannot and should not use baking soda instead of cornstarch when a recipe calls for cornstarch.
Baking soda and corn starch are not interchangeable in recipes because they have completely different purposes in cooking.
Cornstarch is typically used as a thickening agent in sauces and soups, while baking soda is a leavening agent that will help baked goods rise.
Baking soda adds a particular flavor to the recipe and both of them have specific chemical properties which is why they act as leavening agents.
To use them in soups or sauces may not yield the results you want.
Some cornstarch substitutes include.
Potato Starch.
Rice Flour.
All-Purpose Flour.
Tapioca Flour.
Arrowroot Powder.
Xanthan Gum.
Baking powder is also not a substitute for cornstarch.
Since cornstarch acts as a thickening agent in recipes, baking powder does not possess the chemical components to replace cornstarch.
Adding baking powder will create air bubbles but will not thicken the mixture.
To make homemade cornstarch.
Bring the corn into the blender and add some water to cover the corn in the blender.
Blend until you notice a smooth texture.
You can decide to blend the corn in batches if the amount of cornstarch you want to make is much.
Repeat the process until you are done with all the corn in the bowl.