In the case of using an AJAX call to download a JSON file, the JSON file may be part of your project files or be stored online on a distant URL.
In both cases, AJAX is used to download and retrieve those external data and feed them into the array of your project.
The download action of the Array plugin does indeed "transform" the current values and state of the array into a JSON object.
You cannot "download a JSON into the array" using this action.
See the manual article on Arrays for more explanations about those actions.