Do we know where exactly did samudra manthan occur and 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.
Quoting from Srimad Bhagavatam (8.6.22):
क्षिप्त्वा क्षीरोदधौ सर्वा वीरुत्तृणलतौषधी: ।
मन्थानं मन्दरं कृत्वा नेत्रं कृत्वा तु वासुकिम् ॥ २२ ॥
सहायेन मया देवा निर्मन्थध्वमतन्द्रिता: ।
क्लेशभाजो भविष्यन्ति दैत्या यूयं फलग्रहा: ॥ २३ ॥
O demigods, cast into the Ocean of Milk all kinds of vegetables, grass, creepers and drugs. Then, with My help, making Mandara Mountain the churning rod and Vāsuki the rope for churning, churn the Ocean of Milk with undiverted attention. Thus the demons will be engaged in labor, but you, the demigods, will gain the actual result, the nectar produced from the ocean.
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.