Hi El Mehdi, I hope you're doing well!
The process for entering and exiting low power modes in devices implementing the NXP Connectivity Stack can be seen described in the Connectivity Framework Reference Manual document included in the SDK, which can be found in the following path:
<…\SDK_2.2.1_QN908XCDK\docs\wireless\Common\Connectivity Framework Reference Manual.pdf>
Chapter 3.15 describes the functionality of the Low-power Library, and has notes and explanations specifically for the implementation in the QN908x family of MCUs.
Other useful information can be found in the QN9080's BLE Application Developers Guide, in the following path:
<…\SDK_2.2.1_QN908XCDK\docs\wireless\Bluetooth\Bluetooth Low Energy Application Developers Guide.pdf>
Please let me know if you need more information.
Best regards,
Sebastian