Several attributes of demons follow directly from their non-material nature.
* Being made of no parts, they’re effectively indestructible.
* As pure intellect and will, they’re really, really smart and ferociously determined.
Having no physical properties, they don’t take up space.
* As a consequence, they don’t need to “move” or “travel”. Though not omnipresent, they can be anywhere they think about without traversing physical distances.
* Therefore, it’s less accurate to say that a demon can “get inside of” someone/something and better to describe its activity as “attaching itself” to a person, place, or object (remember: a demon is “located” wherever it focuses its thought).
* How can nonphysical beings interact with the physical? Probably not that differently from the way your own spiritual faculties can affect your body (e.g. psychosomatic and mental illness).