Works are said to enter the public domain when they are no longer protected by copyright. There are four main ways a work enters the public domain.
The Copyright Term Extension Act of 1998 extended copyright protection by 20 years and applied to both new and existing works. As a result, works that would have entered the public domain in 1999 had their copyrights extended until 2019, works that would have entered the public domain in 2000 had their copyrights extended until 2020, works that would have entered the public domain in 2001 had their copyrights extended until 2021, and so on.
After a long hiatus, January 1, 2019 marked the first time in 20 years that works entered the public domain due to copyright expiration. New works will continue to enter the public domain every year on January 1st, Public Domain Day.
Almost anything you want! Public domain works can be remixed and reused for commercial or educational purposes. Be careful with your use of collections that include public domain content. As noted on the Stanford Copyright & Fair Use site:
An important wrinkle to understand about public domain material is that, while each work belongs to the public, collections of public domain works may be protected by copyright. If, for example, someone has collected public domain images in a book or on a website, the collection as a whole may be protectable even though individual images are not. You are free to copy and use individual images but copying and distributing the complete collection may infringe what is known as the “collective works” copyright. Collections of public domain material will be protected if the person who created it has used creativity in the choices and organization of the public domain material.
Source: "Welcome to the Public Domain," Stanford Copyright & Fair Use, https://fairuse.stanford.edu/overview/public-domain/welcome/.
The sites listed below represent just a small sample of places you can find content that is in the public domain.