How do astronauts/cosmonauts manage the atmosphere at the International Space Station as their missions are years long, versus shorter flights?
Do they recycle? Import via cargo vessels? Electrolyze water? Chemical processes?
Several approaches are taken.
Cargo vehicles bring up Oxygen and other atmospheric components (Nitrogen, etc).
The Russian segments life support system works different and independant of the US side, which is a nice redundancy model.
The Russian side has a system that recycles water from urine and moisture that needs to be removed to electrolyze to oxygen and hydrogen (and throws hydrogen overboard).
They have a backup system of 'candles' that 'burn' and release oxygen.
The US side I am not as familiar with specifics. But there is a great article on ECLSS at the ISS. ECLSS is Environmental Control and Life Support System.
There are many interaction points between different systems, using by products of others. So it is not just about 'air' or atmosphere, it is also about water, carbon dioxide, nitrogen, and more.