A Real Last-Modified Date, Served From Your Own Domain
A sitemap is a list of your pages handed to search engines, with a little information attached to each one. Your site maintains its own, updates it when something changes, and serves it from your own domain rather than from a shared address.
What is on it, and what is deliberately not
Every page of your site that should be found, and nothing else. External links in your menu are not listed. If you have a link to IMDb in your navigation, imdb.com does not appear in your sitemap, because a sitemap is a set of claims about your own site and listing somebody else's page is not a true one.
Each URL carries a last-modified date taken from that page's own record of when you edited it. A page with no usable timestamp gets no date at all rather than an invented one.
That distinction matters more than it sounds. A date that is demonstrably wrong is worse than a missing one: it teaches a crawler to stop trusting the field across your whole site, so it stops working on the pages where it would have helped. Missing is neutral. Wrong is actively costly.
Two fields that sitemaps historically carried are omitted on purpose. Change frequency is ignored by Google, which they have said publicly. Priority exists to rank your own pages against each other, so setting it identically everywhere is the one configuration guaranteeing it says nothing.
Why your own domain matters
Search engines treat a sitemap as authoritative for the domain it is served from. Serving yours from your own domain means it speaks for your site with no ambiguity about whose pages it is describing.
It also means the file survives you changing anything else. Your sitemap address stays constant whatever you do to your pages, which is what you want when it is registered somewhere.
What to do with it
Mostly nothing. It maintains itself, which is covered in automatic sitemaps.
The one worthwhile action is submitting it once in Google Search Console, which takes about two minutes and gives you a report on which of your pages are indexed. That is the single most useful free feedback loop an actor website has.
Related: canonical URLs and per-page titles and descriptions, both of which make a sitemap more useful than it is on its own. The broader picture is in actor website SEO.
