The servo rail on carrier board does not provide power. You have to supply voltage to it so that you could power any device connected there.
You may check the voltage range from docs.cubepilot:
https://docs.cubepilot.org/user-guides/autopilot/the-cube-module-overview#summary
For the PWM voltage level, you may have a look at this topic:
5v is possible from the cube Orange and yellow. On the IO pins, there is a parameter to switch between 3.3 and 5. All other Cubes and Pixhawks are 3.3v out