How are demons able to come to this world?
They can "fall" like other being.Splitting themselves into lower self(physical) and higher(ethereal).Or they can manifest as shadow,energy,voice for some time.
Are they shape shifters?
They are energy thus they can change form tho they are not immune to physical laws.
why or why not do demons attack humans?
They are higher spiritual being thus they don't attack or posses people.However they are made(one part) of dark energy and many other being(made of dark energy) posses or attack thus they think demon possessed or attacked them.Demons if dragged by some beginner and painfully bind into rune,circle will attack.But anything will.
Demons are ancient,wise teachers and warriors.High spiritual being.Tho made of dark energy some are truly beautiful.