In short the creation, maintenance and dissolution of the material world is conducted by Lord Krsna through the three purusa avatar's.
The first is Karanodakasayi Visnu who creates the total material energy known as the mahat tattva.
He then enters each particular universe as the Garbodakasayi Visnu who develops the universe. It is from the navel of the Garbodakasayi that Brahma is born who is the secondary creator.
Then as Ksirodakasayi Visnu He enters the heart of each living entity and every particle. He is known as Paramatma.