You can build your own online store in the Shopify Store Editor, by picking an ecommerce store theme and then drag-and-drop modules to customize it. If you want to add more features, you can visit the Shopify App Store. There are apps for almost everything you could want.
Users do not have access to the code behind their online store, which means that they are somewhat limited in their options. The design of an online store will depend on themes built by Shopify, however, they can also be customized to some extent. The advantage to not having access to the code is that sellers do not run any risk of breaking essential code. They also run less risk of getting hacked.