It looks like the car has a total of four catalytic converters. You've got two pre-cats - one hanging off each manifold. These pre-cats each have an upstream and downstream oxygen sensor.

The upstream sensor helps to monitor the air/fuel ratio by sampling raw exhaust gas as it comes out of the engine.

The downstream sensor is there just to make sure you've got the pre-cat in place. The ECU is looking for a voltage difference between the up and downstream sensors as the result of the pre-cats being in place. If the voltage difference between those two sensors is out of spec., you'll throw a code.

Further down the exhaust system you've got two, main cataylitic converters. These don't appear to have any sensors near them. These two can be removed without throwing any codes.