Hi Davide, I hope you're doing well!
The sources for the QN9080's bootloader are available as an example included in the SDK archive for this MCU.
It can be found in the following path of the SDK:
<…\SDK_2.2.1_QN908XCDK\boards\qn908xcdk\wireless_examples\framework\bootloader_otap>
Information about the bootloader can be found in the BLE Application Developer's Guide, in the following path of the SDK:
<…\SDK_2.2.1_QN908XCDK\docs\wireless\Bluetooth\Bluetooth Low Energy Application Developers Guide.pdf>
Please let me know if you need any more information.
Best regards,
Sebastian