it's the inner wheel with a tilt/lean. Your weight has to get out over the COG to balance. The feeling ranges from foreign to terrifying and I'm not sure why you wouldn't just go with an excellent cargo bicycle that avoids the entire issue.

You would think so but no. The inner wheel is the one that lifts.

What’s the explanation that this is different from bi-wheelers?

It wants to roll over, just like a car would if it had very sticky tires and took a sharp turn at speed.

The fun bit is that on three wheels it steers like a car, but on two wheels it steers like a bicycle.