Just the hightlights:
The viewfinder for a camera is made to match the view of that camera's normal lens.
If you use a different lens , you need an external viewfinder to show the view of that other lens. There are a lot of different types. The two that I own are: a soviet turret type--this has five individual lenses that are marked for 28,35,50,85, and 135mm lenses. the front of the viewfinder rotates and shows the approximate field of view for each lens. The other one that I own is much smaller, has frame lines for "tele" and "wide"(35mm and 75mm?).
Cameraquest has a pretty good page about this
HERE There are also individual finders for specific lenses, or specific focal lengths, that give a better approximation of the view.
In use, I focus with the camera's VF/RF then compose with the external finder.
There is an astounding number of different types and makers of these and I own just the two so I'm sure other people will have different things to add to this.
Rob