Music and lyrics written by an American author and published in 1922 or earlier are in the Public Domain in the United States. No one can claim ownership of a song in the public domain, therefore public domain songs may be used by everyone. PD songs may be used for profit-making without paying any royalties. If you create a new version or derivative of a public domain song, you can copyright your version and no one can use it without your permission. However, the song remains in the public domain, and anyone else can also make and copyright their own version of the same PD song.
Songs change over time.
Even though a public domain version exists , some versions may still be under copyright protection. The only way to confidently identify a PD version is to find a copy of the song with a copyright date old enough for public domain status. You can then use that PD version or work from it to create your own derivative work. If you work from a version still under copyright protection, the copyright owner can likely make a valid claim for royalties. There are songs written after 1922 which are PD in the US, but only rarely can they be confidently identified without the advice of an attorney or rights clearance agency.