@User standard cells can be simulated with ngspice. However this is slow (although more accurate if you put all the parasitic effects due to routing and coupling). Regarding the standard cell xschem library (for now only the fd_sc_hd lib) when generating a spice netlist the spice subcircuits found in the sky130 pdk of the used standard cells get included into the netlist. There is a test_stdcells.sch example in the xschem_sky130 repository. Notice that in real circuit simulation you should verify your design using netlists extracted from layout, containing all parasitic effects.