We can’t & won’t go into any details because of confidentiality restrictions, but what i can tell you is that in step M3 Apples authentication coprocessor comes into play.
I know, it’s not part of your project but using Nukis BLE API gives much more options (commands, lock states) and is therefore a better way to interact with Nuki accessories rather than relying on HomeKit with its two states and one single command.