I am learning string theory, as I understand, gravitons exist as modes in string excitations, and also other particles. It gave me this picture: a lot of strings fulling in the spacetime, excitations on strings are particles. But QFT gave me another picture that quantum fields exist in spacetime, excitations of these fields are particles, particles can be created from vacuum and annhilated. So, if I consider a finite number of strings flying in spacetime, how come it could describe a picture as quantum fields describes since spacetime regions that strings didn't sweep over are completely vacuum. Do we need string field theory to cover QFT?
Another question is about classical gravity. Given the picture I had about string theory, how explicitly the classical limit of this picture could come out as a total geometry as gravitions pass the interaction of gravity while classical gravity theory is pure geometrical?