Familiar spirits are mentioned in Lev. 20:27, 1 Samuel 28:7-8, 1 Chronicles 10:13, 2 Chronicles 33:6, & Isaiah 29:4. Demons are the evil spirits of the deceased giants, and are called familiar spirits in the Bible because they can appear as someone who you are acquainted with and who look familiar to you. The demon will look exactly like the person who is deceased. So where did the demon get the ability to shapeshift from what he looked like as a giant to look like someone familiar to you. Since the giants were the sons of the fallen angels, they must have gotten the ability to shapeshift from their fathers the fallen angels because their mothers couldn't shapeshift. We know that angels can shapeshift to appear as human men because according to Genesis 19:1-11 the 2 angels that appeared to Lot in Sodom looked like human men to the men who dwelt in Sodom. So demons that are the spirits of giants got the ability to shapeshift from their fathers, the fallen angels. Even the fallen angel Satan shapeshifted to look like a serpent who talked to Eve in the garden of Eden according to Genesis chapter 3. Satan is also called the old serpent, the great dragon and Devil in Revelation 12:9 & in Rev. 20:2.
So fallen angels could very well be shapeshifting to look like the 4 types of so called aliens, the grays, the reptilians, the insectoids, and the Nordics with white hair that look somewhat like humans or they could be hybrids that Satan made by crossing human DNA with some other type of DNA to get these weird looking creatures as the grays, the reptilians, the insectoids, and even the Nordic beings who have human features.
A person that shapeshifts to look like a gray, a reptilian, an insectoid or Nordic would have to be possessed by a demon or fallen angel who entered inside of them and controlling them and causing them to shapeshift.
(This video discusses different people who see a human looking person whose face shapeshifted to look like a Gray, and a man who saw a man who shapeshifted to look like a Reptilian.)