String Matching at Scale

String matching can't be that difficult. But what are we matching on? What is the intrinsic identity of a software component? Does it change when developers copy and paste the source code instead of fetching it from a package manager? Is every package-manager request fetching the same artifact from the same upstream repository mirror? Can we trust that the source code published along with the artifact is indeed what's built into the release executable? Is the tool chain kosher?