Arduino NANO 33 BLE

HI,
I want to use NINA B3x as NANO 33 BLE. For this which model of NINA module will be best suitable.
And how to burn the bootloader into the module

I've deleted your other cross-post @monacos.

Cross-posting is against the rules of the forum. The reason is that duplicate posts can waste the time of the people trying to help. Someone might spend 15 minutes (or more) writing a detailed answer on this topic, without knowing that someone else already did the same in the other topic.

Repeated cross-posting will result in a suspension from the forum.

In the future, please take some time to pick the forum board that best suits the topic of your question and then only post once to that forum board. This is basic forum etiquette, as explained in the sticky "How to use this forum - please read." post you will find at the top of every forum board. It contains a lot of other useful information. Please read it.

Thanks in advance for your cooperation.

monacos:
I want to use NINA B3x as NANO 33 BLE. For this which model of NINA module will be best suitable.

The Nano 33 BLE uses the NINA-B306.

monacos:
And how to burn the bootloader into the module

You'll need an ARM CMSIS-DAP compatible programmer/debugger.
Here's a reasonably priced one that's all open source: https://www.tindie.com/products/ataradov/cmsis-dap-compliant-swd-debugger.

Conect the programmer to the SWD pins of the NINA-B306

Start the Arduino IDE.

Tools > Board > Arduino Nano 33 BLE

Tools > Programmer > ARM CMSIS-DAP Compatible

Connect the USB cable of the programmer to your computer.

Power the Nano 33 BLE (you can do this via the USB connector on the board). The debugger doesn't power the board.

Tools > Burn Bootloader