While studying the NMOS amplifiers in my book I came across operations with differential output voltages and this circuit specifically.I want to understand what are the drain voltages and what is the Vdd voltage in this case?
No way on knowing given the circuit shown. normally there is some load connected to the drains of Q1 and Q2 that help set the operating point. The only thing you know about this circuit is that Id1 + Id2 = I.
This circuit has a current output at the drain. It means that the voltage and not the current is defined at the drains. The voltage depends on the loads connected to the outputs and the supply, but it is not included in the picture above.