It's decided by the person who creates that game, so if you want to start a game you too can choose which setting you want.

However some setups are blacklisted so you can't play them on ranked (because pointfarming), and for each round of the competition there are only six designated setups that can be played at comp level, which are chosen by the previous round's winners.

but what decides its rank? eg. how to create a ranked instead of a quick game

Unranked games (the ones you call "quick games") don't have any heart on the left, like this:



Ranked games have a red heart, and competitive games a golden one, like these two:



Unranked games don't give points, ranked games give points just for your overall score on the lobby and site, comp games give you points for the current competition for a trophy and for the overall for the lobby and site too.