Every organization is different. At my previous employer, the terms we used were:
1. Early board-only mock-up (the board layout is done, but the final enclosure design hasn't been determined). The PCBs are typically fabricated in-house in small quantities.
2. EVT. The circuit boards are good enough for initial board bring-up, but may still have serious hardware bugs. Some products might even have more than one EVT build. If the final enclosure is still not ready, these may be assembled inside the enclosure of a previous model of device. PCBs might be fabricated on a one-off basis in-house, or might be mass-produced in a short run (not sure, and it almost certainly depends on the product and the company).
3. DVT. The circuit boards are good enough for general engineering use, but may still have minor bugs. Some products have more than one DVT build. By this point, the enclosure should be roughly final, give or take. PCBs are probably mass-produced in a short run.
4. PVT. This stage is sometimes skipped if the DVT stage is good enough. At this point, any remaining bugs are believed to be minor enough that they can be worked around in software/firmware. All components are mass-produced using the same production equipment that will be used for final manufacturing (at least in theory).
5. Final hardware, which may or may not be identical to the PVT builds.
Between each of these stages, they'll discover hardware bits that aren't quite right (wires crossed, traces too thin to make proper contact, missing vias through the board, etc.), at which point the boards get reworked with tiny wires soldered on between the pins of various chips. By DVT, you should have few (if any) reworks. By PVT, there shouldn't be any.
I'd expect a preproduction camera body to be an EVT-quality build, with reworks to fix improperly routed traces, an with some parts assembled by hand that should eventually be assembled by machine (possibly after making minor design or layout changes to make that possible), etc. I'd expect the prototype to be built within an existing body, with the intent to swap out the button assembly and associated wiring harness in later builds after the body changes are complete.
If they don't use the body of an existing camera, then I would expect many of the parts to be one-off moulded plastic, created in small quantities in their design lab, rather than fabricated on an assembly line. With that said, I'd be surprised if such hardware made it out of the lab. But maybe Canon does things differently. *shrugs*