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[longpanda]

After install, Ventoy will dived the disk into 2 partitions. and partition 1 is formated in exfat filesystem and also is where to put the iso files.
https://www.ventoy.net/en/doc_disk_layout.html

So /dev/sdc is your USB disk, and /dev/sdc1 is partition 1 /dev/sdc2 is partition 2. Partition 2 is for internal use of Ventoy.
You just need to mount /dev/sdc1 and copy your ISO files to this partition.
If you don't use this USB disk in Windows, ext4 is OK.