Genesis was written for humans, not God. The creation account is written in a frame of reference as if a human were atnding on the earth at that time. The creation account of Genesis si exactly what a man would have seen at that time, with human eyes. As God stretched out the heavens (space) He added energy to space (let there be light), and space absorbed that energy and expanded, like a sponge, and reflected excess energy. I suspect this is the “background radiation’s” source. The light at that time may have been so bright, that the light from the sun and stars may have been blocked for a few days, until the light faded and they could be made out.
Genesis says that a being of infinite MIGHT and MAGESTY took his time and put in the equivalent of a man’s work week to create the universe. He ‘divided light from darkness’ creating the laws of the universe, he brought forth life, and he made humanity and gave them souls after his own image so that they could dwell with him forever in Heaven. Then he rested on the seventh day.