At the highest level:

Fuel supply to the engine is complicated by the saddle tank, so there are dual senders and additional connectors to reach both sides of the tank.

Ventilation is sealed to control evaporative emissions (they are federally regulated). Vents run to a carbon cannister.

To make sure there are no uncontrolled evaporative emissions, there is a small pump that pressurizes the system, and the pressure is monitored. If there is a leak, the SES light comes on. That is what causes the SES light to come on if you leave the gas cap loose.

Others can add more, but that is enough to make it much more complicated than an older vehicle.

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue