"Fish finder" could more simply be termed "sonar". And then within that there are different implementations of the sonar technology. Different frequencies of sonar for different levels of sensitivity and clarity. At each of the websites of the major sonar makers you can read up on all the specifics about how they produce and interpret sonar signals and graphically present that to the user for locating and displaying depth, fish, underwater structure, underwater objects, etc. (Lowrance, Humminbird, Garmin, Raymarine, etc.)
GPS, chartplotting, navigation are all based on signals received from satellites. They can do simple things like show you where you are on the lake, river, or ocean by pinpointing your location on an on screen map (orientation). And some can do pretty complex navigation by integrating GPS data and detailed electronic charts to help you choose a safe and efficient route, or even choose the route for you. And they can guide you along the route with graphical, scrolling on-screen images from various available points of view (navigation).
Many sonar (fish finders) have GPS capability built in. Many chart plotters have sonar capability built in. And in chartplotting it's really all about the maps. Many include rudimentary maps, some include fairly serviceable inland or off shore maps. But quality navigation needs quality maps which cost extra and are typically regional, and frequently come on a memory chip that plugs into the compatible chart plotters. You can find out what maps and mapping software are available from the manufacture's websites of the chart plotters.
Hope that helps a little, but getting the right one for you will take some research on your part. And then learning on your part once you get it.