Hi,
There's no straight answer to how your external server provides the URL.
Chevereto external storage works as a "external disk" in which Chevereto will store the media files. To do that, you use FTP/S3/Etc to configure a external storage provider. For Chevereto, this provider is just a place to store files which later it will "map" to image URLs: Key point here is that each storage provider needs to provide access to the files uploaded there, which is achieved using a URL enabled by your external storage provider.
Keep in mind that it is the external storage provider which provides this URL resolver, Chevereto only maps to the target content uploaded there.