I'm not sure specifically which 'billionaire maid' story you are referring to. There could be various fictional or real - life scenarios that might fit this description. It could be about a maid who works for a billionaire and discovers some secrets, or perhaps a story of a maid who has an unexpected encounter with a billionaire that changes her life in some way.
They can be. Many readers are attracted to the Cinderella - like storylines where the maid, representing the underdog, catches the eye and heart of the wealthy billionaire. It gives a sense of hope and the allure of a fairy - tale romance in a modern - day setting.
One common theme is the class difference. The maid is usually from a lower social class compared to the billionaire, which creates a lot of tension and interesting interactions in the story.