I’ve heard it said that the fallen angels themselves were bound, but the spirits of their offspring with women roam the earth, as demons. The Ethiopic Enoch backs this up. I don’t necessarily accept or reject this explanation. It’s not counter to the Bible but it’s extraBiblical.
Well, the only extra biblical account I put any stock in is the book of Enoch, which is mentioned in the bible, and we have some really early versions (I believe from the african church).
At any rate, it says that the “giants” (and other corrupted offspring of the fallen angels interbreeding with mankind) were destroyed, and it’s their spirits that haunt the earth.
The original angels are chained (restrained) now until the final days (according to the bible)