You need a hacked 3DS (already running custom firmware, usually Luma3DS + boot9strap).

Develop a feature that enhances the bootloader functionality of "boot9.bin" with additional security verification and recovery capabilities. This feature aims to ensure the integrity of the boot process, preventing unauthorized or malicious code execution during the early stages of system boot-up.

The 3DS uses two primary processors: the ARM11 for the user interface and games, and the for security and legacy support. When you power on a 3DS, the ARM9 BootROM (represented by boot9.bin ) is the very first code to run.

You cannot dump boot9.bin without already having boot9strap or another exploit that gives ARM9 access. Follow the official (https://3ds.hacks.guide) to install boot9strap first — the process will automatically dump boot9.bin for you.