While most of these answers are satisfactory, I feel like I should clear it up a bit. Air at higher altitudes aren’t necessarily lacking oxygen content, it’s just that the air is so thin we can’t get enough of it in a breath so we suffocate and die. Passenger planes are constantly taking air in at altitude and pressurizing the cabin. An outflow valve is constantly letting air out at a controlled rate, to maintain a set pressure. So as you fly you are constantly supplied with fresh air that’s been pressurized for breathing, and the plane is constantly farting out it’s outflow valve 🙂
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