Gotta love these "How do I make game X" threads...
A game like that has a budget with a great deal of zeroes - they spend a good deal of that on ads (funny ones, with quality CGI and voice over work) so it's certain the game will get plenty of exposure and thus plenty of players and thus plenty of revenue from IAPs.
So they can afford to have the servers and the decent programmers and the art folk and everything else needed to keep the game running. And when it's been running for a while this happens (Wikipedia):
"The game has been very successful for Supercell. By April 2013, Supercell had only two games on the App Store, Clash of Clans and Hay Day, which together had grossed $279 million. Daily revenue was at $2.4 million, with 8.5 million daily players, and Forbes projected a revenue of between $800 million and $1 billion by the end of 2013. They ultimately earned $892 million (compared to $101 million in 2012)."
Long story short - you probably won't make a game with millions of players that earns BILLIONS on your own, especially if you need to ask these questions. Start sensible, start small.