Is browser cross-contamination between VM qubes possible?

[Lace]

But see, having a persistent VM has now made me wonder how it is deleted when I do finally delete it?

What happens when non-disposable VMs get deleted?

What happens when a VM gets deleted, that was not originally set to be a disposable? Is the data queued to be eventually overwritten or what happens to it? Is there a shred process that randomly overwrites this data automatically for whatever a user deletes including VMs and Templates and iso image files? What happens?