The Guardian and Food and Water Watch have investigated grocery store items, and found that four or five firms dominate the market for many of the items.
Read the investigation here: The illusion of choice: five stats that expose America’s food monopoly crisis. https://www.theguardian.com/environment/2021/jul/18/america-food-monopoly-crisis-grocery-stores?utm_term=d5e2598366af41470835fe08d7eae58c