Google is finding pages on my site that are not listed in my site map. How does Google Search find a list of pages on a site to index?
The sitemap is there, in part, to help Google find pages it might not otherwise locate, such as orphaned pages (pages with no links pointing to them). However, the main method Google uses to find pages is crawling - that's following links on pages. The links may be on your own website, or they could be links from other websites. If the indexed page is not in your sitemap, then there must be one or more links pointing to it from somewhere. Note that a link doesn't necessarily have to be visible on your website - it may just be something in your website source code, such as a meta tag or structured data URL.
I have an old website that I send only the home page to Google and test how Google indexing my other pages.
The result: Google index my first page then search in this page for other links for my website, after that this new pages are indexed too and every links in the new pages for my website are added for next indexing pages, in my Facebook I add other link for hiding page in my website and its also added. Also if you use Google adsense or Google analytics in your pages it will be indexed too.
Same time pages are find by Google but not indexed for same reason like low graphics or similar to other pages from your website or other websites.