FETs are symmetric with regards to source and drain. In other words, they work the same even if you reverse source and drain.

I see, there's no assumption that somehow the body is electrically tied to the source in the spice model of the FET?

The body is an explicit fourth pin that you can connect where you want. In this case VPB for the pfets and VNB for the nfets. When you buy a discreet FET in a 3 pin package, the body and one of the sides will be connected together inside the package and that will break the symmetry

Sounds good, thanks. I assumed that symmetry was broken with some sort of connection between body and source in the

sky130_fd_pr__pfet_01v8_hvt

model itself, but I guess not, this clears it up. Thanks to both of you.

In fact, X1's body is definitely not tied to one of the other terminals. Generally, the circuit you show will work with VPB=VPWR and VNB=VGND. In this case all but X2 have the body tied to either source or drain.