Square uprights to clamp the flat bar against. In this case bolted to the perimeter of the table (see all the holes around the sides).
And lots of shimmying parts in the plane that they don’t have stops, till everything is in exact position.
Those protection pads are ceiling board (cheap flat plaster, ie spatter resistors). Needed depending on welding process. And this is only for tacking.
Table top is plastic wrapped because it had to stay outdoors last night, and this is mist-belt forrest.