Also this whole argument "flat spacetime + spin-2 gravitons = curved spacetime" is just for illustration or calculation purposes and not to be taken literally. I think what you guys do is to assume flat spacetime, then do perturbations to make it arrive at curved spacetime. But all this just a trick, or even figurative. This is because strings could be having 11 dimensional background or unknown background and it producing our gravity or curved spacetime directly from the 11D without any flat spacetime. Do you agree?Yes, in the string picture the strings are always there, but at low energy or large length scale it's a pretty good approximation to replace the string with a particle. Low energy or large length scale means we don't look so carefully, since we aren't looking at fine scales, so we could mistake a string for a particle such as a graviton or electron. In this sense particles "emerge" at low energies or large length scales as excellent approximation to strings.