Fair use is a legal doctrine that promotes freedom of expression by permitting the unlicensed use of copyright-protected works in certain circumstances. Section 107 of the Copyright Act outlines four factors in evaluating fair use of material.
To reuse copyrighted material (such as images or text excerpts) you will need to request and obtain written permission from the publisher or author/creator.
Public domain is a designation for content that is not protected by any copyright law or other restriction and may be freely copied, shared, altered and republished by anyone. There are four common ways that an item will arrive in the public domain.
More information on public domain from Stanford University Libraries
"What is Public Domain?" YouTube, uploaded by U.S. Copyright Office, 18 December 2019, https://www.youtube.com/embed/PMp_-OX15Jc