They all have their own store brands.
At least this is how I understand it. I am 100% open to being schooled in how it actually plays out. I'm just a software guy, but I like to learn.
One tl; dr thing is that the supermarkets are a _supplier_ to both end customers and those with products to sell. They supply shelf space and customer reach, in the very literal sense that companies bid for things like endcap placement (the end of the aisle being better than being in the aisle, since everyone making an orbit through, say, the deli section will pass your goods).
I guess it seems a little obvious in retrospect that there must be a bit of that going on, I've long noticed that some suppliers have a lot of responsibility in the store beyond delivering product. Like the beer guy pretty much runs the entire beer aisle. And it's one of the big names supplying all the beer, not just their own.
I've seen similar action in some stores by the soft drink vendors and even the bread vendor. Non-store employees on the floor putting stock directly on the shelf.