"originally film cameras used a reflector to bounce an image onto film"

This is not true. The mirror in SLR (single-lens reflex) cameras redirects the light coming through the lens to a viewfinder (by way of a prism), through which the user sees what the lens sees. When the shutter is pressed, the mirror flips up so the light travels from the lens directly to the focal plane (where the film or sensor is). Digital SLRs work the same way in this regard. Earlier film cameras were basically empty boxes with a lens on the front and the focal / film plane on the back. Some (commonly known as view cameras, which are still made today) had / have a ground glass panel on the back to act as a viewfinder, in front of which the film holder would be inserted and the film then exposed to the light coming through the lens.