This is all very nice, but has nothing to do with Ventoy.
Ventoy creates a bootable USB stick with 2 partitions.
The ISO are copied into the second partition.
Done.
Nothing has to be mounted. Maybe change the USB port (also with dd).
If the stick does not work with Ventoy, you should traditionally burn the ISO with dd.
Everything that has been presented to you so nicely here is already on the internet and we want to avoid making a copy of all the solutions again in our forum.
Means that we give here help to the self-help with
https://wiki.archlinux.org/title/Dd
I am sure there are also examples here in the forum.
Like
Edit:
sudo ventoy -u /dev/sdd
**********************************************
Ventoy: 1.0.66 x86_64
longpanda [email protected]
https://www.ventoy.net
**********************************************
Upgrade operation is safe, all the data in the 1st partition (iso files and other) will be unchanged!
Update Ventoy 1.0.65 ===> 1.0.66 Continue? (y/n)y
esp partition processing ...
Open ventoy efi file 0x610ac0
ventoy x64 efi file size 1765376 ...
Open bootx64 efi file 0x610ac0
Open ventoy ia32 efi file 0x610f10
ventoy efi file size 1187840 ...
Open bootia32 efi file 0x610ac0
Update Ventoy on /dev/sdd successfully finished.