The emission factors used by Electricity Maps depends on the specific region.
Our default emission factors come from the IPCC (2014) Fith Assessment Report report, and are fully described in our wiki.
For certain areas and zones (all of Europe and US, along with a few other countries), we have more specific emission factors. The actual numbers used (as well as a link to the source) can be found in the config/zones YAML file for each specific zone.
For Europe, the methodology for calculating emission factors is detailed in our wiki page for EU emission factors. The same applies for US emission factors.