USB Passthrough

On any VMs where you want a USB device such as a USB Drive to passthrough to your VM, go to your VM settings page, USB tab and switch to USB 3.0. Do this for all VMs where you want it. If you want all VM's to have this ability, do this on your templates as well. If you don't, you may get a message saying "unable to attach device" whenever you try. If there are still issues on a particular device, try USB 2.0 instead. I've found USB 2.0 to be more reliable with most devices.

Then while running the VM, go to "Devices"->"USB" to see the devices you can pass to the VM from the Host.

Keep in mind if you select "Generic USB2.0" or something similar you may end up disconnecting all internal and external devices including a laptop keyboard from the host. You may have to powercycle your computer to regain control of your computer. Instead select individual devices.

Using a USB Drive is my recommended method for transferring files from VM to VM, or from VM to Host. There is a way to install additional software inside a VM to transfer files, but I do not recommend it. That software can even allow the sharing of clipboards between the Host and VM, but this is a security risk, so instead I prefer using the USB Drive. I know one person who uses Proton Drive to transfer files between VMs. Proton Drive is an external service so I prefer not to use it for internal file transfer.