They are blow molded plastic. This is why they're so much lighter and more flexible than the other doors out there.
Here's a shot of what their rear passenger side door looks like after taking the hit you see above.
View attachment 216784
I should note that the hit was big enough that the upper door hinge slipped its bolts and got pushed in on the B-pillar. Not saying that the door is indestructible or wouldn't ever break but it didn't for us on that obstacle. What I can tell you is that if this were a metal door, it would have a big dent in it. Trust me, we've done it.