OK, please refer https://www.ventoy.net/en/doc_disk_layout.html
Quote:About Part1
By default, Ventoy select exFAT filesystem for the main partition to hold iso files. exFAT has better compatibility on Windows/Linux/Mac and exFAT is suitable for USB stick.
From Ventoy-1.0.11, you can reformat the 1st partition with other filesystem, exFAT/FAT32/NTFS/UDF/XFS/Ext2/Ext3/Ext4 are supported.
It should be noted that, if you choose XFS/Ext2/Ext3/Ext4, the USB will be unavaliable on Windows and can not be used to install Windows. But if you only use Linux that will be a good choice, because XFS/Ext2/Ext3/Ext4 will have better performance in Linux.