The horizontal alu profile are support for the upper bed, there will be similar profiles also on the other sides, but they are not in place yet.
The lower bed will be foldable, tranforming this area to a large garage.
All external corner joints are made/ will be up by internal and external alu angles. The internal angles are what you see vertically in the picture.
I used larger angles for this particular corners since I will be fastening different stuff to them.