Short answers, pulled from the story.
The year 1915 marked the first known novelization of a silent film when publishers released an adaptation of Les Vampires. This French serial ran from 1915 to 1916 and featured a criminal mastermind named Irma Vep.
Film novelizations became especially profitable during the 1970s before home video technology became available to consumers. These books served as the only way for fans to re-experience popular movies outside of television airings or theater rereleases.
Writers must multiply the twenty thousand to twenty-five thousand words of a screenplay into at least sixty thousand words. They achieve this expansion by adding description or introspection to the original material.
Alan Dean Foster wrote the Alien novelization without knowing what the Xenomorph looked like until later in the process. He described the experience as making his own director's cut with unlimited budget and creative freedom.
The first video game to be novelized was Shadowkeep which appeared in 1984. Matt Forbeck became a writer of novels based on video games after spending over a decade writing tabletop roleplaying game books.