Temperature and humidity sensors are usually designed in two major variations.  In one, the sensor acts as a trigger or a switch when a certain level of temperature or humidity is reached, then the sensor is triggered and a specific circuit is activated.  Other more complex versions are used to measure the actual momentary temperature or humidity in the ambient air.  It does not have a trip point; it has a varying analog response to the level of those two atmospheric readings.

A simple temperature sensor relies on the metallic expansion principles of thermodynamics.  As the temperature changes, the metal bends until contact is made or removed from a switching mechanism.  This would imply the desired temperature has been reached.  More complex temperature sensors, especially for electronic circuits, rely on the voltage drop across a transistor to determine the current temperature.  Since the voltage delta of transistor devices is well known as a factor of temperature, it is very easy to determine what the temperature is by measuring base to emitter voltage.

Humidity sensors usually also have a way of measuring the temperature in the air since humidity is the relation of moisture in the air to the current air temperature.  Humidity sensors typically rely on a capacitor to determine moisture content.  The dielectric element between the two capacitive plates can collect water molecules and have the k-value adjusted according to a known pattern.  This affects the voltage in a way that can be sensed and reported.