The United States Geological Survey provides guidelines for citing USGS
data products (as downloaded from 'get_tiles') at:
https://www.usgs.gov/faqs/how-should-i-cite-datasets-and-services-national-map

To cite terrainr in publications please use:

  Mahoney M. J., Beier C. M., and Ackerman, A. C. (2022). terrainr: An
  R package for creating immersive virtual environments. Journal of
  Open Source Software, 7(69), 4060,
  https://doi.org/10.21105/joss.04060

A BibTeX entry for LaTeX users is

  @Article{,
    year = {2022},
    publisher = {The Open Journal},
    volume = {7},
    number = {69},
    pages = {4060},
    author = {Michael J. Mahoney and Colin M. Beier and Aidan C. Ackerman},
    title = {{terrainr}: An R package for creating immersive virtual environments},
    journal = {Journal of Open Source Software},
    doi = {10.21105/joss.04060},
    url = {https://doi.org/10.21105/joss.04060},
  }