Sea level is the reference: 1 atmosphere, about 101.3 kPa. Go up and you leave air behind faster than you'd think — air is light and thins out on a curve (the standard atmosphere model), so pressure halves by about 5,500 m and keeps falling. Go down and it's the opposite: water is about 800 times denser than air, so pressure climbs in a straight line — roughly one more atmosphere every 10 metres, no matter how deep you go.
At the summit of Everest (8,849 m) this model predicts about 31% of sea-level oxygen — close to the real, measured value of roughly a third, though Everest's actual summit pressure runs a little higher than the standard-atmosphere prediction thanks to a persistent high-pressure weather pattern over the Himalayas, which is part of why the mountain is climbable without bottled oxygen at all.
Go the other way to the Challenger Deep (10,935 m down) and the pressure is about 1,086 atmospheres — enough to put roughly 11,000 tonnes-force on a single square metre, which is why almost nothing built by humans can survive there unprotected.
It doesn't run out in percentage — oxygen stays about 20.9% of the atmosphere all the way up. What drops is the total pressure, so each lungful contains far fewer molecules overall, oxygen included. That's why supplemental oxygen at altitude is delivered at higher pressure, not higher purity alone.
Boiling happens when a liquid's vapour pressure matches the surrounding pressure. Squeeze the surrounding pressure higher (as water depth does) and the liquid has to get hotter before it can boil — the same principle a pressure cooker uses. Push far enough (beyond about 2,185 m depth, 22.06 MPa) and water crosses its critical point: the distinction between liquid and gas disappears entirely, and there's no boiling point left to reach.
Around 19,000 m, pressure drops so low that it matches the vapour pressure of water at normal body temperature (37°C). Above that altitude, without a pressure suit, bodily fluids would begin to boil at body heat alone — a hard physiological ceiling for unprotected human exposure, well below the internationally recognised edge of space at 100 km.
Pressure alone doesn't feel intuitive — force does. The same 1,086-atmosphere pressure at the Challenger Deep becomes a very different, very physical number once you multiply it by a porthole, a window, or an eardrum: F = ΔP × A, the same relationship a hydraulic press or a dam wall uses.