VOCs are measured by MOX multi gas sensor. You may read more about MOX sensors here https://ams.com/air-quality-sensors

Atmotube is using CCS801 gas sensor. You can read CCS801 datasheet.

VOC MOX sensor working principle

We utilize a MOX-based (Metal-oxide) sensor in our product. The functionality of a MOX gas sensor is based on the conductivity-change of the gas-sensitive MOX semi-conductor layers at gas exposure, which can be externally measured and analysed. A specific MOX gas-sensitive layer reacts to oxidisable gases with increasing of the layer resistance and to reducible gases with decreasing of the layer resistance. 

In order for device to show appropriate pollution level, it's being calibrated on production in the clean air/ gas mix, so, device already "knows" the resistance level in clean (99%) air and in dirty air and measures air quality around in relation to those two values, which are stored in it's non-volatile memory. In addition to VOC sensor, there's also a separate humidity/temperature sensor to provide a temperature/humidity readings, which is also used for temp/humidity compensation for VOC measurements (gases act differently in a different temperatures/humidity levels).

Please, note, that since it's a relative gas sensor, it measures a gas mix and is not able to detect exact concentration of one pollutant.