I found this on what eCO2 is:

https://ec.europa.eu/eurostat/statistics-explained/index.php...

Re your other questions; yeah I gather there is some experimentation involved, even with out of the box commercial products.

If others have suggestions on chips or products to buy for the household, I would greatly appreciate it! The MH-Z19 looks like an option for me. Commercial products seem too unreliable for the price.