The SCD4x series of photo accoustic sensors from Sensirion is an excellent choice for indoor CO2 measurements and comparable to NDIR sensors. You can see a comparision of the two in a blog post I wrote some time ago [2].
The SCD4x has an built in temperature and humidity sensor and I think this project displays that data. The purpose of this humidity and temperature is to measure these parameters inside the sensor module for compensation algorithms, i.e. CO2 measurement can be affected by temperature, humidity etc. The self heating effect of the SCD is already cmpensated by Sensirion with a default offset of 4 degrees Celcius [3] but the small footprint of the device and other environmental factors, e.g. ventilation could lead to quite large deviations to the real ambient temperature and humidity.
I think in a future iteration, it would be nice if the monitor could log data, either to built in memory or through WiFi. This could give a lot more additional data, e.g. CO2 development during 24h to see the effect of ventilations and the CO2 decay curves etc.
All in all a nice project!
[1] https://www.airgradient.com/open-airgradient/kits/
[2] https://www.airgradient.com/open-airgradient/blog/co2-sensor...
[3] https://cdn.sparkfun.com/assets/d/4/9/a/d/Sensirion_CO2_Sens... see 3.6.1