This becomes more apparent when you actually do it by following the instructions.
When you Add or Edit Product, each product has several screens of settings, most of which are optional to enable various features. The first screen of settings for each product has a Single File Download checkbox, which if checked would mean that product would offer buyers a file download. Checking that box allows you to upload a file that would be provided to each buyer. If you are adding a product for the first time, the Upload Product File button will appear on the last screen of settings, where you would obtain your button codes. If you are editing an existing product and enable the Single File Download setting, the Upload Product File button will appear right there on the first settings screen.