I am creating multiboot live usb with Ventoy and i am wondering if there is a way to test the usb without restarting the pc?

install linux.
install wmware (example debian https://wiki.debian.org/VMware)
check all ISO you would like to analyse/test/evaluate.
Or stay in you window pc and do the same with wmware.

AFAIK in order to use a live usb drive one must configure to to boot from the usb drive/stick and restart the computer with it connected.
Short answer: No.
(if I’m wrong, hopefully someone will correct me)

Then tell (e.g:) virtualbox to boot to the image or USB...

I believe unless there's some type of BIOS thing which allows you to live swap, you do have to reset.

I got the impression there were some real hardware virtual machines, but haven't used one.

A much better choice today is Virtualbox, which is an absolutely free, and full-featured virtual machine monitor which just happens to be backed by one of the largest software corporations in the world – Oracle.

"Today, I find no(!) positive benefit in "dual booting." Today's computing environments are fully-committed either to "containerization" or "full virtualization," and all modern hardware contains full support for it. The software technologies needed to support it are by now perfected – and they happen to be free.

"Virtualbox" runs on everything, and runs everything. And, thanks to the underlying hardware support, the performance of a virtual machine is "almost native."

Test as in boot to usb I assume.

It isn't a documented feature but one can fool with Virtualbox to make it boot to a usb drive. Web pages tell how to do that. Not hard but why don't you do a real test with boot?