In fact, when the Ventoy program is installed on the flash drive, the entire process of creating a multiboot flash drive will be completed, since you just have to copy the ISO images you need to the flash drive (in the usual way), i.e. exactly the ones you want to boot from, such as images of Windows, Linux distributions, or just images of some bootable applications.
Moreover, for convenience, you can create some folder in which to copy all the images, the Ventoy program will search for available ISO files in all directories and subdirectories. All found ISO images will be displayed in the flash drive menu when you boot from it.
The only thing worth noting is that there should not be spaces in the name of ISO files and it is better to name these files in English.
For the test, I will copy the ISO images to the USB flash drive: