I stumbled across this website a couple of years ago, 2012ish, when my daughter remarked that a particular series about cats (ack! sorry, had a hairball there) could not possibly be written by the same person, there were too many of them (books) and they all felt...different, from book to book.
This agency has a standing application for authors to apply to work for them. I don't know the terms, but you generally get paid a flat fee for writing like this, and you don't get to say you wrote "xyz" until much later, but it can depend on the contract your agent negotiates, I guess. I've never done this, so I don't really know. We all know it's done, I had just never seen an agency advertise for ghost writers like this.
It's an intriguing idea, and a way to learn the craft by...well, writing, although you've gotta be pretty good to start. They provide the well-plotted shell, with already established characters, and you flesh it out. They read first chapters from several applicants, then choose the one -- the writer -- they feel does the best for the series, and bango! You're writing a book, and learning what a best-selling series writes like. It's gotta be a great education!
Some authors my daughter really likes got their start here, for example, for that (ack! -- another hairball) series I mentioned earlier (and is still one of her favorites - down to reciting names, clans, etc.) and a dragon series writer who so generously signed all her hard-cover books at the last Tucson Festival of Books. We absolutely adored the Dinosaur Cove series, when my kiddos were much younger and in early chapter books.