Does Supply lead to Demand or does Demand lead to Supply?

It occurs both ways. I must say supply leading to demand 15-20% and demand leading to supply 80%

Initially when a product is launched, because of supply some customers may opt to buy it. But all further sales would happen only when there is a demand for the product. Only when there is a demand for a product, the shop owners would buy them, the stockists would sell them and the manufacturers would make them.

Let us say you want to open a company that manufactures Tooth paste. Assuming you live in a country where people do not brush at all, would you still want to manufacture it? Even if you do, there would '0' demand for your item. So, you may not manufacture it at all... So in any economy, demand drives the supply in nearly 80% or more cases.