Yes, I see many sites submit their sitemap to google use xml format, and there are also many tool for generating sitemap, they are also .xml
Both are important in sitemaps.the site will be crawling the website and generate an XML sitemap for submission to search engines. You can then use your XML sitemap to make a custom HTML sitemap - great for SEO (Search Engine Optimization) and for your site visitors.
xml sitemap for search engines and html sitemap for visitors navigation.. so you need to create both the sitemaps.
xml sitemap is good for search engine to index your site while html sitemap is used for user. Both sitemaps are good. Create xml sitemap using tool and submit it to google webmaster.
Both Sitemap are good. If you have few pages in website then there is no need to put the sitemap.xml in the website.
XML sitemaps are crawled by the search engines and HTML sitemaps are for human. It is always better to use a XML sitemap. It can be easily created using the online sites.
i am using the Google Xml and Yoast SEO both. They both creates really good xml sitemaps, but an expert told me to use only Google XML plugin. As this is more accurate for Google. I didn't follow the reason but since I started using it, issues and error crawls are reduced.
Yes, xml sitemap is visible to google boat whenever google crawls and html sitemap is visible to the end user.