Gasoline is a commodity. There is a federally mandated minimum for detergents and other additives. In most cases these are added at the distribution terminal/tank farm. Depending on location and the size of the terminal, additive packages can be added to a dedicated tank or in the tanker when being loaded.
If you are buying from a local no-name corner store you "should" be getting the federal minimum. If it's a very small retail operation they could be tempted to buy any trash gas a outlaw wholesaler/trucker will sell them for a low ball price.
The major name brands have additive packages above the minimum. The source of the commodity gas of different brands frequently comes from the same terminal with the specific brand package added.
Of course being employed by a major oil company for 40 years I do believe you receive some benefit of using a major brand with their above and beyond minimum additives. Shell, Chevron, and Texaco are my first choices.