A sitemap is an XML file at sitemap.xml listing the URLs a site wants a
search engine to index. It aims to be close to exhaustive: every page that
should show up in search, not a curated selection.
That is the opposite instinct from llms.txt, which leaves most of a site
out on purpose. Birta's own site has one at /sitemap.xml, generated from its
published pages; a project you publish through Birta does not get one of its
own today.
For how a sitemap differs from llms.txt and robots.txt, see
what is an llms.txt file, a real one explained line by line.