Usually, third-party accessory makers do it in one of two ways: they either reverse-engineer the camera’s communication protocol, or they obtain protocol details through a licensing agreement with the manufacturer. Reverse engineering is difficult, can vary by camera model, and is not officially supported.

That said, Nikon’s SDK may already include useful protocol information for USB communication. At least some Nikon SDK packages include MTP/USB documentation describing commands the camera can respond to, which could help if your accessory communicates over USB.

If your goal is only basic remote triggering, you may not need a full SDK or API at all. Many cameras support very simple wired remote control through the remote port, often by shorting specific pins for focus and shutter functions.

So: for advanced hardware integration, there generally isn’t a simple public “hardware API” in the usual sense. Practical options are using the SDK’s protocol docs for USB control, using the remote port for simple triggering, or pursuing licensing if you need deeper access.