The links to S4TF implementations in 01_matmul are broken, since source code has been refactored substantially.
For others searching, firstly there is a nice discussion with some good references in this issue.
The specific references from the notebook:
…Tensor ends up being syntactic sugar for the TensorFlow operator set. You can dive in and see its implementation in Swift in the S4TF TensorFlow module…
→ implementation in Swift in the S4TF TensorFlow module
…here is the basic addition operator, defined over all numeric tensors…