Amazon who also have a really big development team would never put a webview app. But it is still not so complicated, I am almost sure they are consuming a private Rest-Api and all the app is just sliders and modals to show products and change from page to page sending the http requests to the server to get things done.

The items coming from the big retailers , companies and also from small business persons . They make there own shop in the amazon and sell there items online. When item is sold then amazon person take it from them and reach the interested person with items.