Over the past year or so, a tremendous amount of businesses have sprung up that sell exclusively on Amazon. This is mostly due to the success of training programs such as Amazing Selling Machine (ASM) and other similar programs. These programs teach how to select products, source them from suppliers, private label them, and optimize their sales on Amazon.
Those that learn this way typically move from a product, to a product line, to a brand, which can eventually really be considered a business. Of course, only a small percentage get that far. As with all training, various factors determine success, including good product selection and how much effort and experience the business owner has to contribute.
Of those that do fairly well, they end up in a precarious situation. That is, they are almost completely dependent on Amazon for income. That is why they often turn to Shopify.