Even before creation, only one being existed. Lord Vishnu, in His child-form, was lying on the banyan leaf, sucking His toe, in the Ksheerasaagara. He wished for creation. He assumed His 4-armed form. From Lord Vishnu’s navel came Lord Brahma. From Lord Brahma came Sage Marichi. Sage Kashyapa is the son of Sage Marichi. He was married to 13 daughters of Daksha Prajapati, who was also a son of Lord Brahma.
Out of these 13 wives of Sage Kashyapa, Kadru and Vinata were the mothers of the serpents and Garuda, respectively. Out of these serpents, Shesha was the eldest one. But he was very noble in nature. He was devoted to Bhagavan. But his siblings were very bad in nature. So, disgusted by their nature, Shesha left his home and did deep penance for several years. He went to Badarikashrama, Gandhamadana, Gokarna, and Pushkara Kshetras and did an intense penance. Pleased by his penance, Lord Brahma appeared in front of him and asked him to ask for a boon.
But instead of asking for anything temporary, he asked for the permanent devotion to Lord Vishnu and to remain anchored in righteousness. Lord Brahma granted that boon to Shesha, plus assigned him the task of lifting the whole Bhumandala or Bhugola on his hoods, because the Bhumandala was destabilizing at that time. This Bhumandala consists of 14 lokas, such as Satyaloka, the above-most loka, Tapoloka, Janoloka, Maharloka, svarloka, bhuvarloka, and bhuloka. Bhuloka is where we live. Below Bhuloka are Atala, Vitala, Sutala, Talatala, Mahatala, Rasatala, and Patala lokas. Below this Patala loka is Shesha lifting the whole Bhumandala. The serpents in Paatala loka worship Lord Sankarshana, residing within Shesha. This whole Bhumandala is of the size of a mustard seed on the hoods of Sheshadeva.
So, as per the instructions of Lord Brahma, to stabilize the earth, Shesha held up the earth on his thousand hoods. The Bhagavata Purana states that Shesha holds the whole Brahmanda on his hoods, and that the whole Brahmanda is just as the size of a mustard seed. By seeing Shesha’s bhakti towards Bhagavan and his selflessness, Lord Vishnu was very much pleased and thus accepted him as His bed. Shesha reclined on the Ksheerasagara, and Lord Vishnu rested on him. Thus, Shesha became the bed of Lord Vishnu.
Shesha has both, the serpent form, as well as devata form. In his devata form, he is white in color, with a crown on his head and adorned with ornaments and wears Neelambara. He has joined both of his hands in reverence. In the other two hands, he is holding a plough and a dandaayudha (a cylindrical club). Vaaruni Devi is his wife and is seated on his left thigh. This Shesha in his serpent form can be seen behind Lord Vishnu. Within Aadishesha, Lord Vishnu is present as Lord Sankarshana. So, Aadishesha worships the Lord Sankarshana present within him.
Shesha who was holding this Brahmanda (as discussed above in the Bhagavata Purana section), is further held by SriMukhyaprana Vayudeva. Even during the Ramavatara, when Lakshmana was hit by Ravana, he fainted. At that time Ravana tried to lift Lakshmana. Lakshmana at that time remembered his original form which is, Aadishesha form with which he lifted the whole Brahmanda on his hoods. Ravana couldn’t even lift Lakshmana. Hanumanji, the incarnation of Mukhyaprana Vayudeva, lifted Lakshmana easily and brought him to Lord Rama. So, the point is Mukhyaprana Vayudeva lifts Aadishesha, who lifts the Brahmanda on his hoods. And this Mukhyaprana Vayudeva is further lifted by Lord Vishnu, in His Kurma form.
Hope this helps!
Jay SriRam!
ShriKrishnarpanamastu!