The only thing that ACF stores is the ID of the image.
When getting the data ACF calls WP functions so this is what WP is returning. This means that somewhere on your site WP is finding the production URL for the site.
'url' => wp_get_attachment_url( $attachment->ID ),
'link' => get_attachment_link( $attachment->ID ),
https://developer.wordpress.org/reference/functions/wp_get_attachment_url/
