When we read vv. 3-5, we can tell that God is not manufacturing what our physicists call "light," but instead showing that light and darkness both have their function, and that in alternating sequence God is ordering the universe to function according to time. The ancients knew nothing of light in the sense of a particle and a wave. What they could see is that light functioned to give day and darkness functioned to give night, and day and night in alternating sequence give us "time." God was ordering the cosmos to function in an orderly way. So also, on day 4, God is showing that the sun, moon, and stars function to give us calendar and seasons. He is order the heavenly bodies to function in a certain orderly way.

What "light" did God create on Day 1, did this light enable vegetation to grow on Day 3 (when plants were created), and why did He create this "light" just to have it replaced by the Sun on Day 4? -Genesis 1:3-19

"And God said, 'Let there be light,' and there was light. And God saw that the light was good. And God separated the light from the darkness. God called the light Day, and the darkness he called Night. And there was evening and there was morning, the first day." (Genesis 1:3–5)

God created plants and vegetation on Day 3, prior to creating the sun (which He created on Day 4). But He had already created a light source on Day 1 that assumedly fulfilled duties that the sun would eventually fulfill. The heat from this light (or another source altogether) made it possible for the plants, trees, and other vegetation to grow prior to the sun being created. The Sun was created (Day 4) after Plants and vegetation were created (Day 3).

And this light which God created on Day 1 was separate from God himself as He himself created it.

And this light was not the Sun, which He created on Day 4.

So then, what light did He create on Day 1, and why did He create this light just to have it replaced by the Sun on Day 4?