Ignoramus8975 wrote:
I would like to get some clear idea on how to test components of what
I build.

Say, just a servo: can I test it by connecting a small DC power supply
to + and - cables and gradually increasing voltage? I should see it
run, right?

You mean just testing the motor? Yes, a DC supply or a D cell will work.
Or, a servo with amplifier: give amplifier power and apply small
voltage to the +/-10v command input. Then it should move the servo
motor. Right?
If the servo amp has available +/- 15 V output, you can connect a pot
across those outputs and wire the + command input to the pot's wiper.
Make sure the pot is centered before enabling the servo amp.

Jon