The underlying stack (Nearby Connections) is open sourced on GitHub[1]! This stack is what handles the negotiation between Bluetooth/WiFi and chunks the files while sending. There's a couple protos on top of Connections that are specific to Nearby Share, that handle identity and file metadata (eg. mimetype) that aren't a part of this repo. Those are being merged into Chromium first, but may eventually become their own library.

Nearby Share was written originally in Java, so the C++ port is a bit less mature. But hopefully having something out there is better than nothing. If you decide to take a look, do leave feedback. We monitor the GitHub page and I'm active on StackOverflow if you use the google-nearby tag.

[1] https://github.com/google/nearby-connections