Do we know where exactly samudra manthan was done if it ever did go on ?
It is known by the name Kshir Sagar, as explained here:
In Hindu cosmology, the Ocean of milk (kṣīroda, kṣīrābdhi or Kṣīra Sāgara) is the fifth from the center of the seven oceans. It surrounds the continent known as Krauncha.2 According to Hindu mythology, the devas (gods) and asuras (demons) worked together for a millennium to churn the ocean and release Amrita the nectar of immortal life.2 It is spoken of in the Samudra manthana chapter of the Puranas, a body of ancient Hindu legends. It is also the place where Vishnu reclines over Shesha Naga, along with his consort Lakshmi.
Here is an article for info on Seven Oceans in Hindu Mythology
According to Sankhya philsophy, there is Purusha and Prakriti (Spirit and Matter) This Prakriti is the Ksheera Sagara.
From the union of Purusha and Prakriti, the golden womb called Hiranya-garbha is formed(Brahma). From this is born the universe.