You can find the low level RTC functions in the "rtc.h" library file; on my system it is located at
\Arduino\hardware\espressif\esp32\tools\sdk\include\soc\soc\rtc.h

Don't get confused with the other rtc.h that is located at
\Arduino\hardware\espressif\esp32\tools\sdk\include\esp32\rom\rtc.h