Dependent voltage/current sources
A voltage/current depending source in real life needs to be built from actual electronic components. Usually made from operational amplifiers or transistors. They vary a bit from the ideal voltage/current sources in that they:


Don't have infinite voltage/current and have finite power
Usually have small offsets
Have source impedance
Have bandwidth or slew rate (can't source/sink current infinitely fast)


Here are some ways these circuits can be built in the real world:

B and C are voltage dependent current sources

An op amp in a voltage follower configuration is a voltage controlled voltage source.


Source: https://www.maximintegrated.com/en/app-notes/index.mvp/id/3869