As far as I can tell, consumer interchangeable-lens cameras are all categorized as single-lens reflex, rangefinder, or mirrorless. I feel like I never see these classifications used, though, for fixed-lens cameras like the Leica Q, Sony RX1, or Fujifilm X100T. Are all such cameras mirrorless or do some of them use a mirror and prism?
Are all fixed-lens cameras mirrorless?
No. At least Olympus E-10 is DSLR (the mirror does not flip though, it is somewhat similar to SLT). There may be more examples from film era, dunno.
Another example of no.
The dual lens cameras have a mirror. https://en.wikipedia.org/wiki/Twin-lens_reflex_camera
Not to expose the film, but they have one, and normally they have fixed lenses.
Probably all fixed lens cameras are mirrorless. The purpose of reflex viewing (the mirror) is for the viewfinder to be able show exact view of multiple lenses. Today, the digital sensor can also show it.
My opinion is if they don't say Reflex, they are assumed mirrorless. We used to never need that distinction. :)
Twin lens reflex (Rolliflex or Yashica type) are fixed lenses, and are mirrorless in the film path, but the viewfinder second lens uses a fixed mirror. I would call that mirrorless. :)