Unlike many other Android devices (e.g., Google Pixel, OnePlus), Samsung phones do not use a standard “Fastboot mode.” Instead, Samsung devices use Download Mode (also known as Odin Mode) for low-level firmware flashing, unlocking, and system repairs.
If your goal was simply to unlock the bootloader or install custom recovery (TWRP), Download Mode is the correct first step.
Here’s a clear, step-by-step write-up on how to access on a Samsung Galaxy A12 .