As other people have mentioned, quite often the large store contracts an existing manufacturer of the same product to supply an equivalent or identical product.

What hasn't been mentioned is the effect on the manufacturer. It happens a lot that the large store increases their order until the manufacturer is too reliant on them. The manufacturer might even expand their workforce and facilities and take on more debt. And then the large store starts squeezing the manufacturer, knowing that they can't afford to lose the order.

It's most effective for food products that don't travel well. Things like baked goods and ready meals. There's a lot of smaller local suppliers of those kinds of food products. They go through years of slow growth and then they rapidly expand and become trapped.