Only God has the power to create things. But he can have help with the design.That is why they can only be mutants or huge giants when having relations with humans.
The demons have the power to go into the body. They can also use telapathy. This same power is used as a so called abduction which is really a taking control of a body. There is a misunderstanding that there are aliens out there but it is just another demon sad to say.
Now after the flood there were no more nephlems or giants. After 1914 the devil was casted down here and we can see more wars and hunger.
The question I have is what is stopping the demons to create more giants or mutants?
I am sure no one will try to find out since most here want to shift the blame to the Watchtower society for having those giants in the first place.