To cite extrafontdb in publications use:

Chang W, Kryukov A, Murrell P, Bertrand F (2025). Holding the Database for the 'extrafont' Package. R package version 1.1, https://github.com/fbertran/fontcm.

Corresponding BibTeX entry:

  @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},
  }