There is no such thing as "international copyright". Copyright subsists only under the laws of each jurisdiction. There are international treaties pursuant to which each country agrees to make its laws conform to certain standards - but the US blithely signed up to the Berne Convention and to a considerable extent went its own way nonetheless.

The US has a weird and wonderful hybrid system involving registration of copyright. You can find out more at the Library of Congress website. In general under the Berne Convention the duration of copyright must be life (of the author, not the performer) plus 50 years (and usually to the 31st December). The EU made that life plus 70. The US has pfaffed around, going from two 28 year terms requiring renewal to longer terms. The current US position is: -

"As a general rule, for works created after January 1, 1978, copyright protection lasts for the life of the author plus an additional 70 years. For an anonymous work, a pseudonymous work, or a work made for hire, the copyright endures for a term of 95 years from the year of its first publication or a term of 120 years from the year of its creation, whichever expires first. For works first published prior to 1978, the term will vary depending on several factors. To determine the length of copyright protection for a particular work, consult chapter 3 of the Copyright Act (title 17 of the United States Code)."

The usual scam is for an entrepreneur to re-register a slightly altered version of a work and then claim that that starts a fresh copyright term running. Other scams involve asserting that someone younger and longer-lived was a co-author so that his later death is the date on which the 70 years start running.