In other words, by breaking the laws of physics, which is typical for UFOs: flying at thousands of miles per hour without sonic booms, making sudden right-angle turns, crashing and causing destruction without leaving debris. These are real but not physical. They are demons, who have temporarily left their demonic realm (as the Bible documents in innumerable passages).
Foe a scholarly study by a Christian astrophysicist who has personally investigated thousands of UFOs, see Lights in the Sky & Little Green Men: A Rational Christian Look at UFOs and Extraterrestrials.
Glad you posted this. I was going to cite the book myself.
A lot of what I have read refers to demons as the spirits of dead Nephilim (i.e. halfbreeds) whereas fallen angels are, well, just that...spirit beings from another dimension who are anti-God.