Most of them live in Japan. Sumo wrestlers live in a 'stable' where they train and live. They will spend most of their day in the stable eating, training and sleeping.
Anyone can become a sumo wrestler. The first step is to be accepted into a sumo stable, a sort of club or commune where you live and train. You can enter a stable at a young age, as young as 13.
No, there are no weight classes in sumo. So a wrestler can be matched up with someone who is much heavier. That is why weight gain is so important in sumo training.
One of the oldest and most respected is the Tomozuna stable .
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They practice against other sumo wrestlers. Each stable has one or more trainers that will help them work out. Often they will lift weights or push against posts.
They get accepted into a sumo stable. Once there they train for many years and begin their trip up through the ranks.
Sumo is a Japanese sport. There are Chinese sumo wrestlers.
This depends on the sumo wrestler and wrestler in question. However, it can generally be agreed that sumo wrestlers are stronger. (However, many recreational/amateur sumo wrestlers are not strong)
All their lives, starting when they are admitted to a stable at about age 13.
There are Chinese Sumo wrestlers. Those that make the most money are wrestling in Japan.