Google's Gary Illyes said at PubCon last week that Google finds content mostly through links and XML Sitemaps. He said the number one way is through links, crawling links on the web and the number two way is through XML Sitemaps.
Yes, this is obvious but hey, Google for Google to list the number one and two way they discover new content for their index.
It is also a good excuse for me to use a Captain Obvious GIF for a story header image.
Best practices is to make sure Google can discover your links both through normal links on the web and also via Sitemaps. Don't just do one or the other, definitely don't just depend on XML Sitemaps.
Give Google multiple ways to find your content.
Forum discussion at Twitter.