From the website:

• ACNH API is a free RESTful API for critters, fossils, art, music, furniture and villagers from Animal Crossing: New Horizons.
• This started for me as a little side project for me but ended up getting much attraction than I ever thought it would get (~3 000 000 requests per day).
• The API is free and without authentification to make it easy for anyone to start developing on a subject they like.

The original documentation can be found here - this collection & API was created by downloading the Swagger file found there, and using the API importer here to make it available on the Qodex network, all creadit goes to the original API creators. I have no association with the game, company, or API.