Hello Laura, A great deal of composers use the Vienna Symphony Library. These are incredible and vital to get hands on!! There are also some underground libraries purely used by the top dogs, they share these amongst one another when they work together. It is a case of also building up a library of samples which can then be edited and placed with the audio. Having both midi and live playing audio makes the bigger difference for sure. You can get a string player to overdub several times and then harmonise over that; then fatten it up with your own midi sounds. Hope that helps. Best, Julian