Leica rangefinders have had two mounts: Leica screw mount (called LTM), which was used on the Leica I, II, and the various permutations of III like in this thread. It looks just like the later "Pentax" M42 screw mount, just smaller. FEDs and Zorkis are imitators of this era of Leicas.

The second mount, started in the 1950s, is the M mount, which is bayonet. Newer M lenses have had electronics added to them, but the mount is the same. You can also use LTM lenses just fine on an M camera, with an adapter.

As for your second question, you can tell a real rangefinder (as opposed to viewfinder cameras) from the patch in the middle of the viewfinder image. Aligning this patch with the rest of your view focuses the image. Simple viewfinder cameras will have framelines to help you compose, and they may even adjust for parallax, but no patch.