If you don't want Ventoy:

1. Wipe the USB: wipefs -a /dev/sdb
2. Copy the ISO image to the USB: dd of=/dev/sdb if=/path/to/image/linux.ISO bs=1M status=progress
3. Make the data partition in the free space: cfdisk /dev/sdb, don't remove the iso9660 signature, create partition in the free space, and Write.
4. Format the partition with filesystem of your choice: mkfs, cryptsetup, etc.

(everything as root, replace /dev/sdb with the location of your USB)

As is, this only leaves exactly enough for the ISO you are currently working with, sealing the fate of the data partition if you need to swap out the ISO. I suspect there is a workaround in theory, but I haven't gotten around to that yet.

Also see https://github.com/thias/glim, a GRUB-based alternative to Ventoy, albeit with less compatibility.

Edit: this will not work with Windows ISOs and the data partition won't show up in certain versions of Windows, in case anyone is wondering

There's ventoy https://www.ventoy.net/en/index.html

I wonder if you could create a partition as you would.with a hard disk or SSD.

A not totally unrelated stack post:

stack post

Sounds like you need to create two partitions and format them with relevant filesystems

Yes, it's not special. If the partition is big enough, you can just put files on it. If it's not, either expand it or create a new one and just use it like normal.

But keep in mind, depending on the tool you use, it might wipe the whole drive next time you write an ISO.

You can use Ventoy and burn even more isos on it, for example rescue distros, Windows, etc.