I've been using this for a few months, it's awesome! Ventoy is a bootable tool that allows you to boot ISOs, even .VHD & .VDI files created with VirtualBox. Ventoy is flashed to a flash drive just like any other ISO with a couple of differences: The first partition is for regular data with a FAT32/NTFS/Ext4 filesystem (this can be reformatted on its own to your preferred filesystem even afterwards) and a small second EFI partition for all the new Windows 8+ systems that use it now to boot with those (BIOS/Legacy mode also supported too). The partitons are setup this way by design, because stupid Windows doesn't detect any partitions except for the very first one in line if you use your drive on Windows. Chances are you won't see the small EFI partiton in Windows unless you manually mount it.
When you boot Ventoy, it gives you a list of ISOs and other bootable files to choose that is loaded onto the data partition, select and then boot. Since it can also boot .VHD & .VDI files from VirtualBox, you can essentially boot fully installed Linux systems on real hardware (not in Live-USB mode). Since your data partition is just a regular readable-writable file system, you can load other data onto it just like any other flash drive. This is great for especially large flash drives like my 64GB Sandisk, where you can take advantage of all the space without the rest being wasted when flashing a single ISO with DD instead.