Fallen Angels and Demons are two different things. Demons can harass humans but can't form physical creatures.
Fallen Angels can become physical beings, just as God's angels. Fallen Angels were cast down to hell for 70 generations before Noah's flood.
See all the details in the compendium of videos by bible scholar and naval engineer Chuck Missler:
https: //www.youtube.com/watch?v=skSTzkmOeQw