God had conversations with Adam and Eve. They understood that He was the Creator. He revealed Himself to them in a way that they could understand. It was only 6000 years ago. In the big scheme of things, that's not really that long ago. Adam lived for 930 years. He could verbally tell stories to his great, great, great, great grandchildren. Try to imagine that.
The Serpent was illusive. He only spoke to Eve. God allowed the Serpent to speak to Eve.
Satan and the rest of the Spirit world (angels and demons) existed already when God created the Physical Universe (Time, Space, Matter) The Spirit World became intricately intertwined with the Physical World.