Plastic from Carbon Dioxide

Choi, C., Kwon, S., Cheng, T. et al. Highly active and stable stepped Cu surface for enhanced electrochemical CO2 reduction to C2H4. Nat Catal (2020).

DOI | phys.org

This is beyond my skill level.  I mean, where do the "H"s come from? I understand how you can take CO2 and split it into Can and Os. I did an experiment, at school, where we took H2O and split that into Hs and Os. But here they are saying they start with Cs and Os and endup with Cs and Hs. Witchcraft.