Current systems, well, at least the one fitted on 737s since around 2006, constantly generates nitrogen, or rather reduces O2 out of normal bleed air and vents that into the center tank. So you do not need to carry nitrogen, there is more than enough in normal air.
Sadly i don't have any access to the AMM as i'm just a pilot and we don't get that usually. So i cannot post any schematics about that system here. Just know there is a panel in the wheel which shows a green, amber and red light, green means operational, amber degraded or temporarily serviceable, red u/s. Oh, and a placard warning that with the system operational there might be a low oxygen content in there and one should be cautious.
What we get in documentation is only:
Nitrogen Generation System (NGS)
The NGS converts bleed air to nitrogen-enriched air (NEA) during all phases of
flight. The NEA is delivered to the center fuel tank to reduce flammability of the tank. The operation of the NGS is transparent to the flight crew; it does not require any flight crew action to operate the system, nor are there any flight deck indications. The NGS automatically starts operating after take-off and runs continuously through climb, cruise, descent, landing and during taxi for a short period of time. The NGS shuts down after a specified period of time or when bleed pressure is no longer available. The NGS also automatically shuts down during the following non-normal flight conditions:
• Aircraft on the ground and not in test mode
• Either engine is not running in flight
• Fire or smoke detection in the cargo or main deck areas
• Left air conditioning pack overheat
• Center tank refueling valve is open
The fuel tanks are primarily protected by precluding ignition sources; hence
dispatch with the NGS inoperative is acceptable under MEL procedures. The NGS has an operability indicator located in the main wheel well adjacent to
the APU fire control panel.