The public domain is neither a physical space nor a governing body. Rather, it is the vast collection of original and creative work that inhabits the world and is not moderated by copyright law. As per the U.S. Copyright Office,
A work of authorship is in the “public domain” if it is no longer under copyright protection or if it failed to meet the requirements for copyright protection. Works in the public domain may be used freely without the permission of the former copyright owner.
Because each country has different copyright laws, a work that may be in the public domain in the United States may still be copyrighted in Canada, or vice versa.
You have the freedom to use, duplicate, distribute, perform, display, and exhibit works that are in the public domain, even for commercial purposes. For example, a theatre company does not need to seek the rights to perform a play written prior to 1923 and may certainly charge admission.
You also have the freedom to create adaptations or derivative works of a work in the public domain. Examples of these would be the 1991 musical The Secret Garden (based on the novel published in 1911) or The River Bank, Kij Johnson's 2017 re-imagining of Kenneth Grahame's 1908 novel The Wind in the Willows.
In the United States, the medium of a work, its publication date (if the work was published), and living status of the author are the primary factors in determining whether copyright law still applies or the work has entered the public domain.