Why is Lord Vishnu there in ocean of milk and why is he sleeping on the snake?

Hinduism is one of the most ancient religions in the world, today practiced by nearly a billion people in the world. Originally called “Sanatana Dharma” by Hindus, Hinduism is characterized by beliefs in “samsara” (reincarnation), “karma” (all actions have consequences), “moksha” (freedom from the cycle of reincarnation), aspects including the “yogas” and “Vedas” from literary works such as the Upanishads and the Vedas, and the concept of multiplicity, or the ideology that there are multiple gods that represent one divine being. There are some Hindus that don’t necessarily believe in the concept of multiplicity. Instead, they simply pick one god to worship. However, many Hindus believe in the Trinity: Brahma (the creator), Vishnu (the preserver), and Shiva (the destroyer). Vishnu, in particular, was seen as a prominent figure in Hinduism for many generations because of his unique nature and reincarnations, and thus continues to be worshipped today. 

Vishnu symbolizes the preserver, the protector, and the sustainer of the world created by Brahma as well as the law of the Vedas. Compared to other deities, he was believed to have a very collected and benevolent nature with his “central character as guardian, protector, and preserver of the world”. Vishnu is portrayed with having blue skin and four arms, and as dressed in extensive jewelry, flower garlands, a wrapped skirt, and a large crown. In the four hands, Vishnu carries a conch shell, a “chakra” (a discus), a lotus flower, and a club-like mace. Vishnu is also believed to live in heaven known as “Vaikuntha and floats somewhere in the sky above the seven heavens”. Vishnu is believed to sleep in a cosmic ocean of milk, the bed he sleeps on being his serpent Anantha-Sesha. This sleeping form of Vishnu is known as Narayana. Narayana’s consort Lakshmi, the goddess of good fortune and prosperity, massages his feet as he lies down on Anantha-Sesha. 

Why Lord Vishnu resides in the ocean of milk? 

It symbolically means - Lord Vishnu is the all-pervasive Lord who expands into everything. He is the maintainer of the universe and the complete cosmic creation.

The ocean of milk is also a symbolic representation of the cosmos or the milky way galaxy. It also called “Ksheer Sagar”.

In Bhagawad Gita it is mentioned: “Planets, Galaxies, and Stars are Thy body, Divinity is established in Thee alone” ~ Chapter 9 Verse 4.

Srimad Bhagavatam, Book 7, Chapter 4, Verse 17: There is a reference of seven oceans which are oceans of saltwater, sugarcane juice, wine, clarified butter, milk, yogurt, and sweet water. The milky ocean represents absolute truth.

Brahma Vaivertta Purana: Cosmologically the Dvipas and Sagaras depict the entire Cosmos. On the island known as Svetadvipa there is an ocean of milk, and in the midst of that ocean, is a place called Airavati-Pura where Lord Vishnu lies on Ananta.

In some other texts, the ocean of milk is a representation of Super Consciousnesswhich is functioning at the deeper level of awareness.

While some others represent as a period of stillness arising from after the cosmic dissolution. Lord Vishnu reclines over the still cosmic waters on the endless coils of the serpent Ananta Sesha, which represents Time, which is equal to 311,040,000,000,000 human years.

The Advaitic texts give us the below interpretation :

Lord Vishnu - Atman
Ocean - Mind with infinite thoughts
Milky Ocean - Sattvika or pure thoughts
Snake - Ego with its many desires
.
In short “When a person who holds his mind in deep meditation upon his supreme self realizes Vishnu, the all-pervading reality”.

Why is he sleeping on a snake?

The snake or Anantha is most commonly represented as our desires/ego/mind which is ever-changing. It represents Lord Vishnu or God resting on our desires/mind/ego.

Some interesting info :

Shesha is said to hold all the planets of the universe on his hoods and to constantly sing the glories of the God Vishnu from all his mouths
.
He is sometimes referred to as Ananta Shesha, which translates as endless-Shesha or Adishesha "first Shesha". It is said that when Adishesa uncoils, time moves forward and creation takes place; when he coils back, the universe ceases to exist.

He is shown as five-headed or seven-headed, but more commonly as a many thousand-headed serpent.

His thousand hoods represent the divisions of time His name means "that which remains", from the Sanskrit root śiṣ, because when the world is destroyed at the end of the Kalpa, Shesha remains as he is.

In the Bhagavadgita of Chapter 10, verse 29, Shri Krishna while describing 75 of his common manifestations, declares, "anantaś ca asmi nāgānāṁ": Of the nagas, I am Ananta..
.
Lord Shesha is often seen floating on the causal waters of the Garbhodaka Ocean, which exists on the bottom of the universe. Lord Vishnu is thus resting on Sheshanaga as They float on the ocean ~ Srimad-Bhagavatam - Second Canto - Chapter 7