To cite extrafontdb in publications use:

  Winston Chang, Alexej Kryukov, Paul Murrell and Frederic Bertrand
  (2025). Holding the Database for the 'extrafont' Package, R package
  version 1.1. doi:10.32614/CRAN.package.extrafontdb.

A BibTeX entry for LaTeX users is

  @Manual{,
    title = {Holding the Database for the 'extrafont' Package},
    author = {Winston Chang and Alexej Kryukov and Paul Murrell and Frederic Bertrand},
    publisher = {manual},
    year = {2025},
    note = {R package version 1.1},
    url = {https://github.com/fbertran/fontcm},
  }