Most ESC`s (Electronic SpeedControllers) supply the required voltage to supply the receiver and servoes. And as you mentioned: through the cable which is connected between the ESC and the throttle-channel of your receiver.

Remember: Opto ESC`s do not supply this voltage. And there is a max limit of how many servoes you can run this way. Usually the max number of servoes is 3 when you run on 3S LiPo. On 2S you might run 4 servoes, and with 4S or larger LiPoes you need to add an external supply and disable the internal supply from the ESC.