This is a resistive sensor that measures the humidity and I want to know how it works based on its circuit diagram.
This circuit does not measure anything. It's strictly an on/off detector. The sensor (传感器) is clearly a resistive type, sold as a soil moisture sensor. "Humidity" is kind of a bad translation.
When the pot is in the center of rotation, the output will switch with the sensor at approximately 10K.
The typical sensor looks like this (from Aliexpress):
Just two exposed electrodes on a PCB. The soil conductivity determines what the resistance between the terminals is.
There are a number of horrible features of this thing, one of which is that the DC current will corrode off the electrodes in short order. If measurements are made briefly and occasionally it might be useful for some purpose, however there is no means provided for powering down the sensor.
If you want to measure humidity of the air, there are a number of very nice devices that can do that, at varying price points and (in)accuracies. Typically modern ones will have on-board circuitry that translates a capacitive measurement into a voltage or digital signal. They usually also have a temperature measurement, that is useful in itself, and as a means of temperature compensation of the humidity measurement.
Anyway, aside from "not very well", here is exactly how it is intended to work:
Left 104 cap, 1K resistor, LED are supply bypass and power on indicator.
Left 10K resistor is the "reference" for comparison of the sensor.
The 10K pot sets the voltage at which the comparator switches (from near ground to Vcc - 1.5V or so is the useful range). That represents a sensor resistance at switching of a few hundred ohms up to maybe 30 or 40K. The RH 104 cap acts as a filter.
The comparator output is open-collector so the RH 10K acts as a pull-up. The RH LED and 1K resistor indicate when the output is low by turning on. That occurs when the sensor is high resistance (eg. when the soil is dry).
This particular comparator can only sink about 6mA and the LED probably uses 2 or 3mA of that. The source current is limited by the pullup and if you try to source more than about 0.5mA the LED may not go out completely.
This is a 2 state circuit - the output is either low or high, on or off. The IC is a comparator.
The sensor varies in resistance with "humidity". When the comparator inverting input (from sensor) is at a lower voltage than the non-inverting input from resistor divider the comparator output is high and the output LED is off. When the sensor resistance increases until oa- rises above oa+ the comparator output goes low and the output LED turns on.
Many sensors can provide a 0-5 V or 0-10 V output. I'd say that your circuit is intended to work with such device. It clearly won't work with 4-20 mA sensors nor will it work with capacitive or inductive sensors. This circuit might work with a purely resistive sensor, but since the set point voltage can be set anywhere between 0 V and Vcc, the sensor needs to get up to at least 50 KOhm to make the range of the adjustment functional in the upper half of it's range and I am unaware of any humidity sensors that operate like that. Most "bare" sensors are capacitive and need to be used with an oscillator.