On 1 January 2021, the works that were published and protected by copyright in 1925 entered the public domain. These works are now free for the public to use as the public wishes to use them.
The US Copyright Office’s blog says, “The critical role of the public domain in human culture is easily illustrated by the fact that so many new works are based on public domain works, such as the works of William Shakespeare, Jane Austen, Bram Stoker, Louisa May Alcott, the Brontë sisters, and Sir Arthur Conan Doyle.”
Copyright is about protecting not an idea (we are not, after all, the Thought Police), but the EXPRESSION of an idea. For example, there are piles of ways for star-crossed lovers to get themselves into trouble; “Romeo and Juliet” expresses one way; “West Side Story” expresses another way. The idea … star-crossed lovers … is the same, but Romeo never sang “Maria … I just met a girl named Maria …” and Maria never told her man “Deny thy father and refuse thy name!” The two expressions of the star-crossed lovers theme are very different.
The public domain Shakespeare work inspires the musical. The musical is protected by copyright to the extent that the work is actually original expression of its authors and not taken from Shakespeare.