Do priority and changefreq actually do anything?
For Google, no. Google engineers have said repeatedly that both are ignored. The reasoning is straightforward: priority is a self-assessment that virtually every site inflates, and changefreq is a promise about the future that the crawler can verify more cheaply by revisiting the page. Some smaller search engines still read them. Including them is harmless, and it is not worth spending any effort tuning them.
Does lastmod matter?
Yes, and it is the only optional element that does. Google has confirmed it uses lastmod, with one important condition: it has to be trustworthy. If every URL in your sitemap claims a modification date of today, the crawler quickly learns that the field carries no information and stops using it for your site. Set it to the genuine date the content last changed in a meaningful way, and leave it alone when you only fix a typo.
How many URLs can one sitemap hold?
50,000 URLs, or 50 MB uncompressed, whichever you hit first. Larger sites split their URLs across multiple sitemap files, often grouped by content type or by date, and then list those files in a sitemap index, which is a small XML document with a different root element. The index itself can reference up to 50,000 sitemaps, which is enough for any site anyone has built.
Will submitting a sitemap get my pages indexed?
It gets them discovered, which is a necessary step and not a sufficient one. Google decides independently whether a page is worth adding to the index, and a page with thin or duplicated content will sit in the Discovered but not indexed state in Search Console regardless of how prominently you list it. A sitemap is most valuable for large sites, for pages with few internal links, and for new sites with no inbound links, where discovery genuinely is the bottleneck.
Is my URL list uploaded?
No. The XML is assembled in your browser. Your URL list, which may include unlaunched pages, is never transmitted or stored.