If you are just talking DC servo's, start with a motor and an amplifier, without the motion control element,
A DC servo does not in and of itself need an encoder to operate, typically in the past DC servo's had a dc tachometer back to the amplifier for velocity control feedback. The amplifier in most cases requires a +- 10vdc analogue signal to control the velocity, If you search the web on basic DC motor operation, you will find the explanation of the relationship between the armature windings and magnetic field and how the armature windings are fed or commutated.
The encoder is only required when you start using a motion controller and it requires to know exactly what the motor is doing and control it via the +- 10v signal. There are various types of amplifier ranging from the older SCR drives to the later drives which used Pulse Width Modulation, but the motor itself did not change that much.
Modern drives, operating under CNC control, no longer use the velocity loop and tachometer, but operate in the torque or current mode, so the encoder itself is all that is needed for feedback.
Like I said there should be a whole raft of web based information on DC servo's that will give you an in depth information.