The purusa, after creating innumerable universes in the mahat-tattva, entered in each of them as the second purusa, Garbhodakasayi Vishnu.
When Vishnu saw that within the universe there was only darkness and space, without a resting place, He filled half of the universe with water from His own perspiration and laid Himself down on the same water. This water is called Garbhodaka.
Then from the navel of Vishnu the stem of the lotus flower sprouted, and on the flower petals the birth of Brahma, or the master engineer of the universal plan, took place. Brahma became the engineer of the universe, and the Lord Himself took charge of the maintenance of the universe as Vishnu.
Brahma was generated from rajo-guna of prakrti, or the mode of passion in nature, and Vishnu became the Lord of the mode of goodness. Vishnu, being transcendental to all the modes, is always aloof from materialistic affection. This has already been explained. From Brahma there is Rudra (Shiva), who is in charge of the mode of ignorance or darkness. He destroys the whole creation by the will of the Lord. Therefore all three, namely Brahma, Vishnu and Siva, are incarnations of the Garbhodakasayi Vishnu.
From Brahma the other demigods like Dakṣa, Marici, Manu and many others become incarnated to generate living entities within the universe. This Garbhodakasayi Viṣṇu is glorified in the Vedas in the hymns of Garbha-stuti, which begin with the description of the Lord as having thousands of heads, etc. The Garbhodakasayi Vishnu is the Lord of the universe, and although He appears to be lying within the universe, He is always transcendental. This also has already been explained. The Vishnu who is the plenary portion of the Garbhodakasayi Vishnu is the Supersoul of the universal life, and He is known as the maintainer of the universe or Ksirodakasayi Vishnu.
So the three features of the original purusa are thus understood. And all the incarnations within the universe are emanations from this Ksirodakasayi Viṣṇu.