Today I had a silly fight with my colleague. My friend told me, "Googlebot crawls a web page only through the links present in webpage or through the number of page views (by text ads)." But I argued, "Googlebot can crawl the webpage even if we don't submit a sitemap.xml in Google Webmaster Tools or don't have any links in the page. That is, Googlebot can crawl the page without any external force, it would automatically visit the page and crawl it."
What's the exact answer? Please inform me of any other factors involved.