There is no single founder/messenger/prophet of Sanatan Dharm. It has many people known as Rishis who contributed to it over time. It is based on divine revelations experienced by a series of Rishis.Sages, while they were in deep intense meditation. Under this state they have the ability to see past, present and future and see events that have occurred and wrote it down for the benefit of humanity. For example, the Ramayan was written by Maha Rishi Valmiki who had the ability to see what actions were performed by Bhagwan Ram during his incarnation on Earth as he recorded all the events in the epic Ramayan. He started to write the Ramayan after Bhagwan Ram was made King of Ayodhya. In same way other scriptures were written by sage Ved Vyas. From Vedas, Upanishads, Ramayan, Mahabharat, Purans, etc… Were all written down by sages (Rishis) in the same way. So Sanatan Dharm didn’t have a single founder but it had numerous founders who contributed to it through out history.