It's aluminium profiles with square section 23.5 mm section specially designed to fit each other through special inserts designed to create joints at 90°, 180° or in various combinations such as variable angle couplings or joints.
I have found these joints in a specialized hardware but I think this thing, with various brands, we are everywhere.
Now, I have not found it necessary to describe the phases of construction in words because the images speak for themselves, and then my English (taken from a simple copy and paste of Bing translator,) is probably not the best.
So I'll just say a few tips on editing:
1) possibly useless hint: measurement precision. I, having taken the salient measures by my two laptops I did a design on paper with measures to prevent more possible errors (which, however, there have been the same)
2) to insert the profile joints I helped with a hammer as they are very hard to insert and virtually impossible to remove; to avoid damaging the work I used pieces of wood on both sides to cushion the blow.
3) each piece that has been sawed (I used a jig saw because they have other things) has been carefully deburred and eventually worked with the grinding wheel to fine-tune the perpendicularity of the cut, which in any case is not critical since the same joints shall ensure "straighten" profile in the right way.
4) the sequence that I wrote is obviously not mandatory.
As regards the last link of this first phase, I think it's the gentlest and Board of control the skis of two tubes above and below before arriving at the bottom so you can adjust some throw in final stage.