Q. A greenish-blue substance has appeared on the copper product.

That is copper rust, known as verdigris. It is harmless, so please use it with confidence.

Q. How should baking soda and citric acid be used differently?

Citric acid, as the name suggests, is mildly acidic and is effective against alkaline stains (such as water scale and soap scum). Baking soda is mildly alkaline and is effective against acidic stains such as grease and sebum. It works in the opposite way to citric acid.