When to generate a sitemap by hand
Most frameworks and CMSs can build a sitemap automatically, and that is best because it stays current. A hand-made sitemap is right for small static sites, landing pages, or a quick fix while you set up the automatic one.
After uploading, add a line Sitemap: https://yoursite.com/sitemap.xml to robots.txt and submit it in Google Search Console and Bing Webmaster Tools.
Questions
Should I include changefreq and priority?
Google ignores both. They are optional and harmless.
Which URLs belong in a sitemap?
Only canonical, indexable pages that return 200. Leave out redirects, noindex pages and duplicates.
What is the limit for one sitemap?
50,000 URLs and 50 MB uncompressed. Larger sites split into several files listed in a sitemap index.
Is my data stored?
No. Sitemaps are read or built for the result you see and not kept.