Series books are often written under pseudonyms by a variety of authors, such as:
Little Animal Arc by Lucy Daniels published by Hodder
Beast Quest books by Adam Blade published by Orchard Books but created by the fiction packager Working Partners
Rainbow Magic Fairies by Daisy Meadows also published by Orchard Books created by the fiction packager Working Partners
Fiction packagers come up with the ideas in-house and they will write a detailed outline saying what needs to go in every chapter. They then have a selection of authors on their list who they will ask to write the first three chapters. They often ask more than one author and choose the one they like best.
Their favourite chapters are presented to publishers and if the publisher likes them, they will commission the writer to write the proposed book in the series or maybe four of the desired books. They will then go through the same process again for the next idea for a story in the series. So each book is written by a different author.