Copyright is a form of protection grounded in the U.S. Constitution and granted by law. It is meant to encourage creative work by guaranteeing the creator of the work the right to profit from that work. It protects creative works of original authorship* that are fixed in a tangible medium of expression*. Copyright protects such works as literary, dramatic, musical, and artistic works, including poetry, novels, movies, songs, computer software, and architecture. Copyright does not protect facts, ideas, systems, or methods of operation, although it may protect the way these things are expressed.
*Original authorship: The original creator holds the copyright permissions exclusively unless:
-they created the work collaboratively (in which case all authors/creators own the copyright)
-they signed those rights away, either through dedication or in a publishing agreement
*Fixed in a tangible medium of discovery means that it has to be physically located somewhere (it can't be an idea, for example, unless it's written down somewhere).
For more information, see https://www.copyright.gov/help/faq/faq-general.html
"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."
-Stanford University Libraries, Copyright and Fair Use
https://fairuse.stanford.edu/overview/public-domain/welcome/
How does something arrive in the Public Domain?
There are four ways that this might happen.
1. The copyright expired : Copyright has expired for all works originally published in the United States prior to 1924.
2. Copyright never protected a particular kind of work. It simply doesn't apply, so that work defaults to the public domain. An example of a kind of work that is exempt from copyright regulations is a phone book. It conveys factual information with very little or no creativity.
3. The copyright owner decides to formally dedicate their work to the public domain, thereby releasing their copyright to the work
4. The copyright owner did not follow the required guidelines to copyright the work. Items published between 1924 and 1989 are especially relevant here, as the publisher was required to formally apply to the United States Copyright Office and then post a copyright statement on the work itself.