Simply put, American copyright law states that once the copyright of a film expires, it then enters what is known as the public domain, meaning that it is common property. Unbeknownst to many, these films are incredibly easy to find on video streaming platforms like YouTube, or even as MP4 files across the web. That is, so long as you know which films to find (and it is worth noting that these are predominantly classic and silent films). There are lists of accessible titles by using information from copyright catalogs, like this list from MUBI and the website Public Domain Movies.
However, as the Pratt Library very straightforwardly puts it, "there is no definitive 'slam dunk' certainty or official list when it comes to the question of whether a film is in the public domain," and therefore these lists may only serve as a general nod in the right direction. This is because public domain status must encompass the copyrights of all the film's individual elements (cinematography, literary works and other source texts, et cetera). Furthermore, restored or remastered versions of the films can often be beholden to copyrights of their own, which are separate from the public domain status of the original. For instance, Ida Lupino's 1953 The Hitch-Hiker may be considered a public domain movie, but it was also recently released by Kino Lorber on Blu-ray, which places the title into trickier territories depending on whether one chooses to watch the newest version of the film, with all of its added (and copyrighted) modifications.
Therefore, to search the web for a public domain movie is much like a mine for gold. Among many duds in a minefield of narrow legal parameters, there is a real gem, and often one that is crucial to the history of cinema. For instance, William Dickson and Thomas Edison's Dickson Experimental Sound Film (1894-1895) is not only available as part of the public domain, but is also the first film with live audio. Dickson's film, an experiment in synchronizing image and sound, was produced at Thomas Edison's laboratory/film studio as a test for Edison's Kinetophone. So, let the search begin!