The leading search engines, such as Google, Bing and Yahoo!, use crawlers to find pages for their algorithmic search results. Pages that are linked from other search engine indexed pages do not need to be submitted because they are found automatically. The Yahoo! Directory and DMOZ, two major directories which closed in 2014 and 2017 respectively, both required manual submission and human editorial review.
Search engine crawlers may look at a number of different factors when crawling a site. Not every page is indexed by the search engines. The distance of pages from the root directory of a site may also be a factor in whether or not pages get crawled