@Link068, the “whole” circuit is obtained when you combine both circuits. This sensor has elements which vary their resistance based on the concentration of some gases. In order to operate the sensors need to be heated above a specified temperature. Thus the circuit on the left. The FET in that circuit, as @ScruffR pointed out, allows you to “accelerate” the pre-heating of the NOx element by allowing more current to flow through the heating resistors (82 ohms) after which it can be disabled for normal operation. The CO sensor doesn’t seem to need this.
The right side of the (whole) circuit creates a voltage divider between each sensing element and a resistor of no less than 820 ohms. The voltage is measured at the junction of the element and the associated resistor. As you may have deduced, the voltage at each junction is proportional the concentration of the (reactive) gases on each sensor.
So basically, you heat up the sensors to their operating range and read the voltage on the output of each sensor. FYI, many resistive type sensors operate this way.