In the .pm3.spice files, there are the threshold voltage values but defined with another variable (such as "sky130_fd_pr__ nfet_ 01v8__ vth0_diff_1) that doesn't seem to be defined in the file. Is there a way to find that value? Or is there a document with some of the general transistor parameters (threshold voltage, mobility..)?

See this file https://github.com/google/skywater-pdk-libs-sky130_fd_pr/blob/f62031a1be9aefe902d6d54cddd6f59b57627436/models/all.spice and this one https://github.com/google/skywater-pdk-libs-sky130_fd_pr/blob/f62031a1be9aefe902d6d54cddd6f59b57627436/models/parameters/lod.spice

@User Thanks for the resource! Do you know what the difference is between variables ending in lkvth0_diff, kvth0_diff, and wkvth0_diff? In the spice file I was looking at, it only had vth0_diff, so I'm not sure which variable that goes to

Probably, they are variables for VT variation as function of transistor dimensions, like L and W. But what you must really be looking for is how it affects a transistor in a simulation. You can simulate any transistor inside a circuit in ngspice, and then show the actual transistor model values, after all those parameters are calculated by the simulator. Try the show command. https://sourceforge.net/p/ngspice/discussion/127605/thread/54ae7420/?limit=50