One main distinction is that Shopify shops have a direct to consumer relationship. They can send you emails, and build up a brand. Amazon keeps the customer relationship, and if you succeed, they'll try to commoditize you. So it isn't that much different for the user (with a unified checkout out cart), but is much better for the store owner.

Edit: you might find this article about the differences between the Shopify and Amazon ecosystems interesting

https://www.nytimes.com/2020/11/24/magazine/shopify.html