On iPhone and iPad, there isn't an exit game button, the game is closed with the same method as other apps.

You can swipe up from the bottom of the screen to minimize the game (you may need to swipe up twice if it's in game mode). You can also swipe from the bottom of the screen and hold your finger in the centre to view all open apps, swiping up again from here will let you force close the game.

Either option is effectively closing the game.

Further information can be found on the Apple Support page: https://support.apple.com/HT201330