Based on YouTube robots.txt, the sitemap used by YouTube itself is a Sitemap. Granted, this may not be the only sitemap you use, just the ones you list in your robots.txt file.
Notice how the sitemap points to the subdirectory "yt". This subdirectory is an access to other YouTube sites such as "YouTube for Developers" and "YouTube Advertise" (with its own non-styled sitemap). You also do not bother to list the regular website that is so popular and popular. Honestly, the sitemap seems a bit random, unless it's a bigger strategy. Maybe they did not sustain it for a while.
An XML sitemap does not mean that everything in it is definitely indexed (this is due to the search engines). It also does not mean that what is not in it is not indexed. It is a suggestion for search engines to pay special attention to certain folders and files.
If you create an XML Sitemap, create it in the root directory (or near the root directory in an accessible folder), and list your most important pages (home page, category pages, etc.), the search engines will follow and index the contents of these folders ( like single videos, posts, etc.). Just make sure that these contents are noindexed / noofollows anywhere else in your preferences, and the Sitemap should be sufficient (or more, depending on your structure and strategy).