How are science fiction movies made?

That's called green screen keying technology, that is, let the actors perform on the green or blue background first, and the technicians use 3d and CG technology to do special effects on the computer, and then cut the blue or green screen background (so-called keying) and match it with the special effects that have been done to express the sci-fi scene. The blue-green background is used instead of red because the blue-green wavelength is convenient for keying.