Textbooks should be created in a simplified way in order to provide ancillary (supplementary) materials for instructors and to help them in the teaching process.
Some open textbooks come with ancillary materials like quizzes, slides, instructor materials, homework platforms etc. Look around on the page hosting the open textbook for links to instructor resources.
Other companies have begun to offer materials for an additional cost that integrates with chosen OER. These are often not OER, but are low-cost solutions that can give instructors the capability to offer the types of features that sometimes come with traditional publisher textbooks.
Some ancillaries can be integrated into a course management system. Your institution may offer technology support to instructors seeking to integrate these materials into learning management systems. Here are some examples: