How do you code a featured image into an XML as a jpg link, instead of using a data-id number?

You have to post two "items" in the xml. The first item is the "parent" portfolio-item... and the second is the attachment linking to the parent item. See below the entire XML rss. (I excluded all the information within the "item tag" which was not specific to this question to keep the answer light and easier to follow).

After writing this, upload the XML file using WordpressDashboard>>Tools>>import or (http://yourSite.com/wp-admin/import.php) Make certain you import checking the "Download/Import images" checkbox.

<rss>

<channel>
    <title>Portfolio-item Name</title>
    <link>http://example.com</link>
    <description>xxxxx</description>
    <pubDate></pubDate>
    <language>en-US</language>
    <wp:wxr_version>1.2</wp:wxr_version>
    <wp:base_site_url>http://example.com</wp:base_site_url>
    <wp:base_blog_url>http://example.com</wp:base_blog_url>


    <generator>https://wordpress.org/?v=5.8.3</generator>

<item>
<wp:post_id>1111111</wp:post_id>
<wp:postmeta> 
        <wp:meta_key><![CDATA[_thumbnail_id]]></wp:meta_key>
        <wp:meta_value><![CDATA[2222222]]></wp:meta_value>
</wp:postmeta>
</item>

Simultaneously add the attachment item which links via the id(number) you assign: In this case im using 2222222 as the image id.

<item>
<title><![CDATA[xxxxxxx.jpegName]]></title>
<wp:post_name><![CDATA[xxxxxxx.jpg]]></wp:post_name>
<dc:creator><![CDATA[]]></dc:creator>

    <link>http://example.com/?attachment_id=1111111</link>
    <wp:post_id>2222222</wp:post_id>
    <wp:status><![CDATA[inherit]]></wp:status>
    <wp:post_parent>1111111</wp:post_parent>
    <wp:post_type><![CDATA[attachment]]></wp:post_type>
    <wp:attachment_url><![CDATA[http://example.com/xxxxx.jpg]]></wp:attachment_url>
    
    <wp:postmeta>
    <wp:meta_key><![CDATA[_wp_attached_file]]></wp:meta_key>
    <wp:meta_value><![CDATA[xxxxxxx.jpg]]></wp:meta_value>
    </wp:postmeta>    
</item>


</channel>
</rss>

Hope this helps. It was difficult to find this, but I sorted it out, hopefully it will help another newbie like myself.