OpenGarage is completely open source, you can find the circuit design at:
https://github.com/opengarage
I am not sure what you mean by ‘the voltage output of the relay’ — the relay does not output any voltage itself, it’s a switch. The on-board relay is rated 3 amp contact current. If your water pump exceeds 3 amp, you should NOT use the on-board relay but you can use an external relay with larger current rating, or solid state relay.