Good morning to you,
First of all, have you downloaded the version of EasyPopulate (EP4) associated with this thread? It has an autoinstaller that takes care of all the install portions when the files are loaded to the admin folder (not sure if the previous statement of uploading to the WebStore meant to your catalog or not. The files are available from:
https://www.zen-cart.com/downloads.php?do=file&id=2069
As to your last question, the purpose of downloading a file is to observe the configuration and arrangement of the file and data so that you can see what needs to be entered where to get the desired results. The instructions for EP4 identify what is necessary to consider when adding product, but no you don't have to re-upload existing product if you don't want to/nor have changes to push to the database.
Also, it is suggested that as you first start using it, that you try working with an existing product just to understand how it works, what happens, etc... (learn) and then do as you wish to manipulate your store.
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