Except the Los Angeles basin is open to the ocean for a good portion of it. And the other side has a valley that leads to the IE, where pollution seeps and sits.

It’s the complete opposite of the two other examples in that list.

It’s the same with Calgary, Alberta as well. Rocky Mountains on one side and the prairie on the other. It still gets inversions and bad pollution.

My understanding is it’s a layer of cold air that sits over warmer air on the ground.

Inversion means warmer air is above cooler air - it's inverted from normal where temperature decreases with altitude and leads to a very stable atmosphere with little convection. So the air on the ground stays there.