I suspect they use something like a TXV (thermostatic expansion valve) that is found on newer household AC units.

This acts as sort of a regulator for the system, which ultimately ends up reducing the load on the compressor as the evaporator is colder.

So while the compressor runs at one stage in the sense of it being directly driven by the engine, the resistance to spin the compressor varies based on the load of the AC system.

that's valid, but even then, there's also the efficiency of whisking away heat from your body to consider, which is the more direct/relevant efficiency concern here.