AP style uses the following rules:
In singular and plural nouns that don’t end in s, add ’s: the author’s book, the children’s toys
In singular nouns that end in s, add ’s: the virus’s spread, the bus’s route
In plural nouns that end in s, add only an apostrophe: the authors’ books, the professors’ partnership
In proper nouns that end in s, add only an apostrophe: James’ coat, Odysseus’ journey