Any flat surface and you've got a pitch. Preferably grass or artificial turf but concrete can be fun too, just not as fun if you fall. You could also look into indoor facilities in your area.
In terms of the world, the best place I've ever been to see a football game was the Millenium Stadium in Cardiff. The atmosphere was incredible. Similar at Old Trafford. So I would imagine those places rank high on professional players favourite grounds as well (or they would if I was a pro).
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If you are referring to "they" as anyone, then all around the world. Soccer is most popular in England which is where it originated and it has slowly grown in the United States. More than half of the population in England plays soccer not just as a past time but as a profession too. In the next few years or decades soccer should grow continuously in the United States.
hey this is difficult to tell, but the answer is "Europe!" and in Europe countries like England, Spain, France, Germany, Italy,etc..., have good domestic leagues with richest clubs like manu,asrenal, Chelsea, Liverpool, Madrid, barca,milan,munich,lyon,etc..,.these clubs offer good money to players so the best players will play in these clubs.
It can be played indoors, but its often either played outdoors on grass, or turf.