If you provide a filename for the ORT format model, a file extension of '.ort' will be inferred to be an ORT format model.
If you provide in-memory bytes for the ORT format model, a marker in those bytes will be checked to infer if it's an ORT format model.
If you wish to explicitly say that the InferenceSession input is an ORT format model you can do so via SessionOptions, although this generally should not be necessary.