Which voltage is lower, 0V or -1V for connecting a MOSFET's source terminal?
A MOSFET in its purest form is a four-terminal device, and the body is not shorted to the source.
In this case we don't say that &quot;the NMOS source is connected to the lowest voltage&quot; we say &quot;the terminal with the lowest voltage (of the source/drain pair) is the NMOS source&quot;.
So, leaving out the gate and body, which are physically different, which ever NMOS terminal is connected to the lower voltage is the source, and whichever terminal is connected to the higher voltage is the drain.