What's the equivalent of 5vSB on a laptop?

Since laptops use only one DC current (19v or 10-12v) what is their equivalent of standby power? How does a laptop restart itself if it has no PS_ON to pull low, unlike a desktop?

Your question's premise is wrong:

Since laptops use only one DC current (19v or 10-12v)

First, these are voltages, not currents. But: Your desktop PC uses one voltage, only, just as well, by your definition, 220 V or 120 V AC.

That's really just the voltage at the power supply entry. None of the electronics within your laptop (aside from the battery management) work at 20V; none of the electronics within your desktop PC work at 220V. In both cases, there's a cascade (literally, dozens to hundreds) of voltage converters chained from that source of power. They produce voltages like 5 V, 3.3 V, 3 V, 2.5 V, 1.8 V, 1.2 V, 0.9 V…, which are actually needed by the electronics. That's the same on an ATX-standard desktop PC, a laptop computer, a car, a smartphone, or a AWS billion-dollar datacenter. You never need the voltage you get from the power source, you need some stable, low voltage very close to the chips in your machines. So you use voltage converters.

Some of them are running while your computer is in standby. And some of these supply power to the components that can wake up the rest.