Debian uses the GRUB boot loader to load the kernel and start the operating system by default. During the boot process, you may be prompted to enter your username and password at the login prompt on your computer.
To modify the boot process, you can edit the GRUB configuration files, which can be accessed by pressing the "e" key in the GRUB menu.
Alternatively, you can modify the boot order and other settings by accessing their computer's BIOS or UEFI firmware.