I recently started implementing custom ops, and I agree TF C++ is a lot harder to grapple with that the Python side, and the current state of the docs does not help at all.
That said, grep or Search in VS Code is your ally: searching class TensorHandle in the TF repo, you can pull up tensorflow/cc/experimental/base/public/tensorhandle.h and see that the Tensor Resolve(Status* status) from which you could then get the buffer?