Apple Books automatically creates samples for all books (except read aloud books). What the sample is made up of depends on the type of book.
If you do not want Apple Books to generate the sample automatically, you can create a separate, custom .epub file and deliver it for use as the sample on Apple Books. Note that not all tools support this option. If you contract with a third party to manage your content delivery, they should be able to do this for you. Contact your delivery company directly for more details.
Note: A purchased book replaces its sample in the customer's library. Be sure to include all sample content in the full book.