[fluffdaddy]

This is impressive, but it isn’t clear how to place the cross pieces on top of the vertical columns. Did I miss something?

[Rio]

I don’t think he’s gotten so far as to place a cross-piece - at least not in this video.
He probably has a plan, though.

[Stoutcat]

I’m going to guess he’ll use the same teeter-totter method as shown in the video: raise one side a bit, slide another board under it; raise the other side, slide a board under that side; back to the first side, and so on. It’s really very ingenious.