Measure volt, amps, watt and kWh of an external 12 V DC system
Fundamentally, the "kind of hardware [used to] measure the volt, amps, watt and kWh and send the value to my RPi" is an ADC. To measure volts and amps you measure voltage across a parallel and series resistor respectively. Watts and kWh are calculated from that.

But this a big undertaking from where you are. Better to get something that does most of this for you. In this case, look for a "DC power meter IC". Internally they're an ADC, but they take care of a lot of the details for you.

Consider, for example, the Microchip PAC1720 or the Maxim MAX44299 or the Texas Instruments INA219. These are not product recommendations, but advice on what to look for. The typical application circuits in the INA219 datasheet in particular should be enlightening.

From there you can go significantly easier again by finding a breakout or demonstration board for these types of ICs. Adafruit's INA219 High Side DC Current Sensor Breakout (product ID 904) is a great example.