How do they evaporate into the air? https://www.epa.gov/septic/types-septic-systems "conventional system" is the one I know most common, and I've never seen an evapotranspiration system.

Ah:

>Evapotranspiration systems are only useful in specific environmental conditions. The climate must be arid and have adequate heat and sunlight. These systems work well in shallow soil; however, they are at risk of failure if it rains or snows too much.

So it's a desert thing, I am not in a desert.

I am in the desert, and still haven't heard of this. Everyone out near me has a leach field.

That is what the Leach field does.

Leach fields are not uncommon: https://en.wikipedia.org/wiki/Septic_drain_field

Leach fields evaporate water.