I am not quite sure what you are asking about ... using Carloop or Building a simulator, or attaching an extra peripheral to Carloop?
Carloop takes power from the 12V OBD2 port to power itself. Since automotive power is not clean at all, Carloop (hardware version 2.3) uses a RT8258GJ6 adjustable buck regulator to provide 5V to power the Carloop electronics up to 1.2A. There are also a few other components on the board to help protect itself.
There is also a CarloopXL (hardware version 3.x), which is not in production, that uses a NCP117 voltage regulator to provide 5V to power the Carloop electronics with enough capacity to power a microSD card (which take quite a bit of power).
In theory, CANbus is a differential signal which means you do not need the automotive 12V or Gnd as references for the CAN Hi and Low signals. Following that theory, you could power Carloop from another power source, but I have not tried it yet. If you are providing an alternate power supply or adding any extra peripherals, then you need to keep track of your power requirements. Remember, Carloop has to Power the Photon (or Electron or RedBearDuo), the CAN transceiver chip, a GPS if it is plugged in, plus anything else you might add. Also, keep in mind that the power supply circuitry is never going to be 100% efficient either.
Did I answer what you needed? If I missed your point, please clarify and I can try again.