Hi,
form files are always stored on disk under the GUID name by default. So, one option would be using a global event handler for the online form object and save the file in the after save event into your custom location, basically duplicate the file. Or, you can do te same basically on the online form web part level as shown in this example. Or, you can try following the approach from this older article (API- namespaces may have been changed since then)