This is a simple method to customize a Debian Installer image, for example in order to preseed the installation. The method is viable if you want to install Debian on a EFI system booting from a USB stick, which should be a common enough use case.

First create a FAT32 filesystem on the removable device and mount it. The device is sdX in the example.

parted --script /dev/sdX mklabel msdos
parted --script /dev/sdX mkpart primary fat32 0% 100%
mkfs.vfat /dev/sdX1
mount /dev/sdX1 /mnt/data/

Then copy to the USB stick the installer ISO you would like to modify, debian-12.2.0-amd64-netinst.iso here.

kpartx -v -a debian-12.2.0-amd64-netinst.iso
# Mount the first partition on the ISO and copy its contents to the stick
mount /dev/mapper/loop0p1 /mnt/cdrom/
rsync -av /mnt/cdrom/ /mnt/data/
umount /mnt/cdrom

# Same story with the second partition on the ISO
mount /dev/mapper/loop0p2 /mnt/cdrom/
rsync -av /mnt/cdrom/ /mnt/data/
umount /mnt/cdrom

You can now edit the files on the USB stick at will. For example, you can add a preseed file called /mnt/data/preseed.cfg and add preseed/file=/cdrom/preseed.cfg to the kernel command line in /mnt/data/boot/grub/grub.cfg as follows:

linux    /install.amd/vmlinuz vga=788 --- quiet preseed/file=/cdrom/preseed.cfg

When you're done, run umount /mnt/data and remove the USB stick. You should be able to boot from it to install Debian normally.