Created: 2017.04.10
I just checked w/ our Website Builder team regarding your specific question, and here's what they said: "Website Builder automatically creates a site map for all published websites but does not automatically submit it to search engines. For customers who update their website using the SEO wizard (on the Business Plus plan) we also do automatically submit the site map to the search engines on their behalf."
The sitemap is available at http://www.domain.com/sitemap.xml
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Then it's just a matter of submitting this to Google via the Search Console: