The voltage Transducer LV 25-P
I just checked the first page of the datasheet, but it seems ot be a pretty standard Hall sensor.

Whatever you want to measure (voltage or current) creates a current in the primary coil of a transformer.

This current creates a magnetic flux inside the transformer core.

A Hall effect sensor measures this flux.

An "opamp" (not really an opamp, rather a closed loop control circuit) observes the output of the Hall sensor and outputs a current into the transformer's secondary coil, trying to cancel the magnetic flux inside the core. This works all the way down to DC. At equilibrium, the flux inside the core is zero, thus the current in both coils is inversely proportional to the number of turns. This is where the ratio comes from. The closed-loop control circuit will always adjust the secondary current to keep the flux at zero.

RM measures the current in the secondary coil and turns it into a voltage that you can use.