Where to put your sitemap
Upload sitemap.xml to the root of your site, so it opens at yoursite.com/sitemap.xml. Add the Sitemap line to robots.txt so every search engine can find it, and submit the address in Google Search Console and Bing Webmaster Tools to have it read sooner.
Do lastmod, changefreq and priority matter?
Google uses lastmod when it is accurate and ignores changefreq and priority. Bing and some other crawlers still read them. If you include lastmod, update it when a page really changes; a date that changes on every page every day tends to be ignored.
Which pages belong in a sitemap?
List the pages you want found in search, using the exact address you want shown: the same https or http, the same www or not, and the same trailing slash style throughout. Leave out redirects, error pages, pages marked noindex and duplicates. A single sitemap can hold up to 50,000 addresses.