Where does the oxygenated cabin air of commercial airliner come from?

Dear Capt. Lim,

Can you explain where the breathable cabin air of a commercial airliner comes from?

I know that for passenger comfort, the cabin is pressurized to 5000-7000 feet atmosphere but cruising at high altitudes (FL180 to FL400) where oxygen is very thin, where is the adequate oxygen needed for breathing comes from?

Is it coming from oxygen tanks tucked away somewhere below deck?

Are any stale air jettisoned or do they just keep recycling?

Does cabin air come in from engine intake or through separate air intake?

Thanks for your answers.

Regards,

James