[wahern]

They manually add headers for spanning openings and leave them in place. You can see here: https://youtu.be/is2UVodNphY?t=37

That said, wooden and, more recently, foam forms have been used since forever. (At least a hundred years?) Breaking the forms away from cured concrete doesn't seem to be problematic.

EDIT: Also, in another video there's a brief shot where the machine is printing the internal zig-zag pattern over a steel mesh manually placed on the wall, presumably for reinforcement. I fast-forwarded through 4 or 5 videos and it's almost suspicious the way they avoid capturing these details. I guess such details don't fit the narrative they're selling.

[walrus01]

They really show only very brief glimpses of the actual printing process. Not even 1/100th of the linear extruded length of a single layer before the video cuts to something different.

[parineum]

I think that answers my question of how it's seismically sound.

There's got to be something in there that supports horizontal forces.

[wahern]

FWIW, they show the mesh in the first few seconds of this video: https://www.youtube.com/watch?v=wCzS2FZoB-I

Personally I think they should emphasize the manual labor requirements. One of the strongest criticisms of 3D-printed and prefab housing technology is that it reduces demand for low-skilled labor. For the past 80 years home construction has been a major pillar of the American labor economy; likewise for China and many other countries. Free or reduced cost housing is great, but to the extent it comes at the expense of a sustainable labor economy, then it's problematic.