Ok, so there is this specific website percentagecal (dot) com

It has a lot of traffic but it doesn't have that much sites linking in. So I was trying to figure out why this site has so much traffic.
Now I noticed it almost has 2 million pages indexed with google. So it seems like there is a page for each possible percentage calculation (in a certain extend of course).
But yet I can't seem to find a sitemap and the 6 calculation samples on the homepage are always the same (not dynamic upon refresh or something).

So now these are my questions:
1. How does google see (and index) all these pages? Without sitemap, dynamic calculations on the homepage, etc.
2. Do you think this website has such great traffic due to these indexed pages?

Let me know your thoughts.