There is no founder/messenger/prophet of Sanatan Dharm. It is based on divine revelations experienced by a series of sages, called Rishis, while they were in intense meditation. Under this state they have the ability to see past, present and future and see events that have occurred in other universe (Parallel Universe). 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 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.