What do the sensors do? There's not much you can change in the catalytic converter so I assume it's just reading temperature? So I assume it's changing the fuel/air combustion ratio according to the cat's temperature?

Not a motorhead but IIRC a combustion with too little oxygen produces soot (pollution) and one with too much oxygen produces NOx pollution, with a sweet spot in the middle. The exhaust oxygen sensor allows the ECU to adjust the air/fuel mix to hit the minimum pollution spot, instead of estimating it.

There might also be a catalyst temperature sensor or something.

It's not a "whole bunch" of sensors, it's a few sensors and it's not some inscrutable magic, it's somethijg someone could replicate in open-source if they had equipment and time. We really need to get away from the mindset that proprietary stuff contains inscrutable magic. It's often worse quality than the open thing. However, it does have the right connections to be allowed to be put in a car that drives on the road.