Think of a Servo as a geared DC motor, that requires POS & NEG voltage, plus a Signal (pulse string), using PWM.
All the Servo driver does (can be something as simple as a 555 Timer IC) is produce the Signal, to enable the Servo to move, either to a preset or defined point, or freely between any two given points (Start point & End point).
A Servo driver can be set up to produce a specific pulse to move the Servo to a defined position, once this information is put in place it's permanent, until as and when the User changes it, now all that is required is a method to trigger (start) the Servo to move, can be anything, Switch, Reed, TTL Logic Signal, PIR Sensor, Hall Sensor, so on and so forth.
Home Brew DCC Arduino Decoders, and Home Brew Servo drivers, precise, infinite, unlimited movement of a Servo or multiples of Servo's can be achieved with a minimum of hardware (electronic components) and as the control program (Sketch) is completely under User (Human) control.
The Arduino UNO has 6 pins ~3, 5, 6, 9, 10 & 11 are able to produce a PWM Signal that can be used for Servo's, below are a couple of simple ideas for use with Servo's, and the Arduino IDE programming environment, simple or complex, single or multiple Servo or Servo's.
Unlike commercial Servo Drivers or Servo Testers where you are restricted on what you can do, no such restriction with the Arduino.
Careful thought needs to given to the design of the voltage regulator circuit to drive the Servo or Servo's which is clean electrically noise free, for a voltage range of 4.8 V to 6 V DC voltage.
Servo's can be very finicky on less than stable power regulator circuits.
Servo's are heavy users of both Voltage and Amp, and don't forget to tie the GND's from any external source to the Arduino.