Idle valve control tends to be of closed loop operation.
So the further the idle strays from the required value the more the ICV opens to control it.

So if you are somewhere with less pressure (high altitutude) it would automatically compensate for that by being open a bigger percentage of the time.

Ignition as idle speed control is crude but some systems do it.

you basically set an "idle screw" of some sort to allow enough air in to idle on both a cold day or a hot day, and then you set the ignition to control the speed.


The way this works would be something like

500rpm 15 degrees
1000rpm 12 degrees
1500rpm 5 degrees

What this means is that if the revs increase, the ignition is naturally pulled further and further down as the ECU interpolates towards the higher rpm values which are less ignition.