Most fish finders work with pretty basic transducer technology. Transducers are what makes sonar possible. Some transducers are sensitive enough to take sonar readings with nothing but their on-board technology, while some, like the Venterior VT-FF001 require you to drop the transducer into the water on an electric cable.
The sonar reflects off anything in the water, as well as the bottom of the lake so it can tell you how deep it is.