To fetch product data in an eCommerce PWA, the app asks a server for information about products. This is done using special requests called API calls. The server sends back details like product names, prices, pictures, and availability. The PWA then shows this information to users. To make the app faster, it can save some data on the device so it loads quickly next time, even without internet. Also, it only loads a few products at a time instead of everything, which helps the app run smoothly and faster.