Yohanes Nugroho at TinyHack writes about reverse engineering Pokemon GO Plus:

Pokemon GO Plus, (which I will refer from now on as PGP) is a wearable Bluetooth Low Energy (BLE) device to be used with the Pokemon GO game for Android or iOS. There have been many attempts to clone this device, but only Datel seems to figure out the algorithm, while the other clones are cloning the exact hardware and firmware.
I will explain the complete certification algorithm that I obtain from reverse engineering a PGP clone, and then I will explain how I did the reverse engineering and how you can extract your own blob and key if you want to clone your own device. I am providing a reference implementation for ESP32 so you can test this yourself.

The article talks about the history of cloning and goes into great detail on the process. It does not discuss generating keys or the Bluetooth blob (as that would be proprietary to the company and could have Niantic ban someone’s account).

https://tinyhack.com/2018/11/21/reverse-engineering-pokemon-go-plus/