An XML Sitemap, in simple terms, is a list of pages on your website. It can be useful for telling Google and other search engines about pages on your site they may not otherwise discover. For most websites, however—especially those on Squarespace—an XML Sitemap is unnecessary. Squarespace employs a clear site architecture and clean URLs, so search engines should have no problem crawling and indexing your content.
While we don't feel it's necessary, if you do wish to create an XML Sitemap, you can find more information here:
When you are ready to add this to your site, you can upload the file to your site from your Site Manager > Data & Media > General File Storage, using this guide.
You can then use the URL shortcut feature to rename this and remove the /storage portion of the URL, see this guide.
As a note, this XML Sitemap will not automatically update with changes made to your site. You will need to regenerate and update the file whenever any structural changes are made to your site.