As far as I know RAPL is implemented entirely in the CPU and is an estimate of CPU power using a complex model of CPU state, temperature and such. I don't believe it's an actual power measurement like e.g. SVI telemetry is.
This was true for earlier implementations, but newer ones actually measure power. There is an ADC in there. At least for Intel. Not sure about AMD implementation.
Doesn't this vary from part to part based on how integrated the VR is in that generation? It seemed to go back and forth in some prior generations, or seemed more accurate with Xeon than with client parts. I still treat it as accurate enough but I have wondered.
