Where are the source codes of external library

I’m current study on how to optimize gRPC and use the optimized gRPC in TensorFlow. I installed my TensorFlow by pip (i cannot install TensorFlow from source becuase my devices’ file system is NFS, which has some problems with bazel). By command ‘pip show tensorflow’ I locate the directory of TensorFlow. Inside this directory, I find grpc’s header files in include/external/com_github_grpc_grpc/. But these are all header files. Where are the cpp files?

Thanks, and please forgive me if this is a stupid question.