Thanks. I've replied to ubuntuforums. Concerning the first attempt,instead,I have created the folder iso_unpacked_and_modified and inside of it I have placed the folder isolinux with the file isolinux.bin. This is what happened :
script2.sh
orig_iso=debian-live-11.5.0-amd64-xfce.iso
new_files=debian-live-11.5.0-amd64-xfce
new_iso=debian-live-11.5.0-amd64-modified-xfce.iso
mbr_template=isohdpfx.bin
# Extract MBR template file to disk
dd if="$orig_iso" bs=1 count=432 of="$mbr_template"
xorriso \
-outdev debian-live-11.5.0-amd64-xfce.iso \
-volid d-live \
-padding 0 \
-map /home/ziomario/Scrivania/PassT-Cubic/ISO/iso_unpacked_and_modified / \
-chmod 0755 / -- \
-boot_image isolinux dir=/isolinux \
-boot_image isolinux system_area=/home/ziomario/Scrivania/PassT-Cubic/ISO/isohdpfx.bin \
-boot_image any next \
-boot_image any efi_path=boot/grub/efi.img \
-boot_image isolinux partition_entry=gpt_basdat
# ./script2.sh
432+0 record dentro
432+0 record fuori
432 bytes copied, 0.000496522 s, 870 kB/s
xorriso 1.5.4 : RockRidge filesystem manipulator, libburnia project.
Drive current: -outdev 'debian-live-11.5.0-amd64-xfce.iso'
Media current: stdio file, overwriteable
Media status : is written , is appendable
Media summary: 1 session, 1310720 data blocks, 2560m data, 40.3g free
xorriso : WARNING : -volid text does not comply to ISO 9660 / ECMA 119 rules
xorriso : UPDATE : 2 files added in 1 seconds
Added to ISO image: directory '/'='/home/ziomario/Scrivania/PassT-Cubic/ISO/iso_unpacked_and_modified'
xorriso : FAILURE : -indev differs from -outdev and -outdev media holds non-zero data
xorriso : NOTE : -return_with SORRY 32 triggered by problem severity FAILURE
Mario.