The first factor is the Mammalian Dive Reflex. When your nose and mouth are submerged your body automatically does some tricks to reduce your need for oxygen. Your heartbeat slows, certain nonessential organs slow way down, parts of your brain slow down, etc. With zero training or preparation, you can almost certainly hold your breath much longer under water than on land.
Then, they also do a lot of training and preparation. One thing they do is hyperventilating. This is *not* something you should do without training! They breathe rapidly for a short time to saturate their blood with as much oxygen as possible before diving. Some may even do this with a higher oxygen mixture from a cannister.
Finally, as others have pointed out, they train to be aware of their body and suppress the *feeling* of needing to breathe. That feeling doesn’t come from your body detecting a lack of oxygen, it comes from your body detecting a buildup of carbon dioxide. With the other tricks, they have more oxygen so they know they can stay down longer when if their body is telling them to breathe.
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