You guys need to remember us in the general public can have what to you are stoooooopid misunderstandings about how aircraft work. For example, I was presuming that once the cockpit closes, the aircraft is completely sealed from the outside, so as to be able to be pressurized for while flying at high altitudes. It doesn't occur to me that a closed cockpit is like a regular car with the doors closed, because a regular car isn't meant for flying at high altitudes.
I think there is a lot you can work out for yourself, which is the point guys were trying to make. A lot of the "stoooooopid" you refer comes from people who just don't care enough to research the answers, which are readily available. Since you appear to care more than the average Joe, we are encouraging you to research it, just like you've obviously done with other subjects, like nuclear energy.
Having said that, with the engines off, the cockpit can get pretty stuffy with the canopy closed, particularly on a hot day- just like your car would sitting outside in a parking lot. Hence the similarity.
If I ever have to sit in an airplane with the engine(s) off, I typically leave the canopy or the door open- the best equivalent to having your car windows rolled down. If I have to close the canopy for some reason (rain, engine blast, etc.), I'll either start the engine(s), or leave the canopy cracked a little to allow some airflow- most airplanes have a method to do this. A little planning goes a long way.
Water. Yeah most high performance airplanes have AC. So does your car. That doesn't keep you from getting thirsty on long drives though, does it? It can also get hot under all that glass, despite good AC flow. Wearing 40lbs of survival gear, maneuvering under g, and high-workload tasks like tanking and CAS... the list goes on. Conditioned bleed air (airplane AC) is also incredibly dry, having been heated up from engine compression and then cooled and re-expanded in an air-cycle machine (most jets) or directly cooled by a vapor-cycle air conditioner (smaller props), either or all of which pulls water out of the air. I flew a 6.5 combat sortie without water one time. It took me almost two days to recover from the dehydration.
Oxygen. OBOGS is the modern way, which is basically an oxygen filter to concentrate O2 out of conditioned bleed air and deliver it to the regulator that feeds the aircrew mask(s). The nice thing about it is it doesn't require frequent servicing, at the expense of some mechanical complexity. Older aircraft use LOX (liquid oxygen), which is a bottle of liquid oxygen under immense pressure (liquid O2 goes a lot further than gaseous oxygen). As it it used, that liquid boils off as a gas and goes through two regulators, not all that different from SCUBA gear, which ensures it's delivered to aircrew at the right temperature and pressure. When the LOX tank runs out, you have an emergency bottle of pressurized gaseous oxygen, or cockpit air to fall back on. Same, if OBOGS malfunctions. Both sources remove nearly all the water from the air you breathe, enhancing the dehydrating effects of flying.