Quite complex, really for what they do.
The basics are this. The whole seat heating is fed from the ignition switch. There's a contact which is switched on with ignition, but switched off again when starting the car, to relieve load on the battery.
This feed goes to the switch to supply the electronics, and also directly to the seat heaters.
The other end of the seat heaters is connected to the seat heaters switch, and the switch connects this to ground, using electronics, to switch the heater on.
The electronics in the switch do a couple of things. If the voltage is low, the heaters are switched off so they don't work when the engine isn't running, and in any case, they switch off after a time interval.
There are no external relays involved in this, so the heat switches have two big terminals to carry that current.
Inside the seats, attached to the heating element, there's a thermostat. When the temperature reaches a preset level, the heating is switched off, regardless of the switch being on. Some people have changed the thermostat for a higher temperature version because they don't think the standard one gets warm enough.
I'm not sure of the actual current taken by the heaters, so can't advise on that.