If you have a custom domain Medium Publication, finding the sitemap.xml is pretty difficult. Generally most sites submit their sitemap to Google’s Search Console in order to better index their sites.
After Medium discontinued custom domain publications, almost all support for custom domain publications essentially stopped.
But what you’re looking for is essentially here:
(your domain)/sitemap/sitemap.xml
For example, this blog has the sitemap hosted here:
https://blog.shawjj.com/sitemap/sitemap.xml
Adding your sitemap to Google Search Console:
And verifying that it has indeed been submitted:
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