The relationship of CO₂ sensing to COVID safety can be found at a number of places, for example https://makezine.com/2021/05/17/the-what-how-and-why-of-co-monitoring/ (and more at https://makezine.com/tag/co2-sensors/ ). Briefly CO₂ levels measure the amount of human breath in your vicinity, which is a reasonable proxy for the number of particles of an infectious airborne pathogen potentially in your vicinity. There are some caveats, mostly related to indoor air filtration systems which can potentially remove (very small) virus particles but not (much smaller) CO₂ molecules, but it's still a reasonably interesting parameter to monitor when you're in a new or unusual setting.
There are four parts to this project:
* A PCB to interface the Sensiron SCD40/41 sensor to the watchy: https://github.com/cscott/watchy-co2-pcb
* A modified case for the Watchy: https://github.com/cscott/watchy-co2-case
* Source code for the Watchy watch face: https://github.com/cscott/watchy-c4
* And, of course, the open source Watchy itself! https://watchy.sqfmi.com/
Lots of details on github at those three repositories, and on SQFMI's page for Watchy.