To answer the first question, we turn to Jude 1:6 “… and the angels which kept not their first estate, but left their own habitation …”. This says that angels descended to earth. The sons of God were angels created by God, and the daughters of men were women born through Adam under sin.
The sons of God or the fallen angels manifested in physical bodies to interact with the daughters of men. This is seen in Genesis 6:4 “… when the sons of God came in unto the daughters of men, and they bared children unto them …”. They could not have been spiritual beings, but had manifested themselves as men. The children of this unholy union between the sons of God and daughters of men created giants known as the Nephilim, which were destroyed in the flood.