"The term “public domain” refers to creative materials that are not protected by intellectual property laws such as copyright, trademark, or patent laws. The public owns these works, not an individual author or artist. Anyone can use a public domain work without obtaining permission, but no one can ever own it." -- Welcome to the Public Domain - Rich Stim
The timing for when a particular type of format of work enters the public domain varies widely depending on a variety of factors including but not limited to:
Copyright Term and the Public Domain in the United States - Cornell University Library
Check out the following sites for resources in the public domain:
From the Public Domain Review at the Open Knowledge Foundation, this guide has links to sources for Public Domain works of many types, such as images, texts, videos, and more.
Images from the collection of the Smithsonian Institution that are in the Public Domain.
Public domain audiobooks read by volunteers.
Images from the New York Times that are in the Public Domain.
A listing of items from Wikimedia Commons that are in the Public Domain.
Books in the Public Domain from Project Gutenberg.
Thousands of short films in the domains of education, advertising, and industry.
University of Pittsburgh Library System. (2021). How Can I Find Public Domain Works? https://pitt.libguides.com/copyright/publicdomain