What is public domain?
Answer
Creative works that are not protected by copyright are said to be in the “public domain,” meaning it is material that everyone is free to enjoy, share, and build upon without restriction. Generally speaking in the U.S., works published more than 95 years ago are in the public domain. There is no easy method to determine whether a work is in the public domain because the laws are complex and have changed many times over the years. There are many websites that explain the details of what is in the public domain to help you decide what you can use, such as:
ALA Digital Copyright Slider
https://librarycopyright.net/resources/digitalslider/index.html
Center for the Study of the Public Domain, Duke University https://web.law.duke.edu/cspd/
The Public Domain Review
https://publicdomainreview.org/