All folders generated by the plugin can be found at the following path:
wp-content/uploads/yith-additional-uploads. If you want to group all the folders with the users’ files in one parent folder in this position, just enter the name in the “Upload folder” option.
All the files sent by users are stored and sorted either based on their ID or the order number they are associated with. You can select one or the other in the option called “Storing mode” available in the plugin General settings.