I have watched videos on YouTube showing the view through the M9 viewfinder, and it appears that only a small window in the middle allows you to focus. However, I once tried out an old Voigtländer rangefinder, and it looked as if the entire viewfinder consisted of two images, laid above each other.
I tried to search the internet for more information about these differences and attempted to find out what cameras have which type of rangefinder. However, I couldn't find anything, probably because I don't know the technical expressions.
Therefore, I ask you: What types of rangefinders exist and what differentiates them? I would also be very thankful if you could explain to me which type of rangefinder, in practice, is more useful for what purpose.