I'm confused about a difference in the NMOS and PMOS. The scenario I'm confused about is when the source is VDD and the drain is connected to GND and output.

For the PMOS, the output toggles between 1 and 0 (opposite the gate) as expected. However, for the NMOS, the output is always 0.

I don't understand why GND pulls VDD down to 0 for the NMOS, but not the PMOS.

Oh, the drain should only be connected to the output, not drain aswell (irl this would kill one's chip, I'll add a feature to show short-circuiting). on pmos the source should be vdd, on nmos the source should be gnd (this doesn't apply for some later levels, but does for earlier ones).