How google crawl a new website that its url has not been manually submitted to google?

I know how google do crawler indexing using sitemap. Google may be able to indexing using link in its website that he crawl. I also know google will fetch your website faster if you submit your website url to "submit url - google" feature. What I'm curious is how google fetch your website if you don't submit your website url and there are no other website that has external link to your website? It looks like google able to do that, but how he do that?