WPC decking on steel structure - substructure unclear

First of all, thanks for all your responses, that has already helped me a lot. Now I have one more question: What do you do with the door niches? (visible in the photo on the right edge) Also install WPC, although there is no proper substructure there – you would have to do some makeshift work. Or are there already ready-made solutions for such purposes that should be used here? Regards N

also place WPC in the niches, with a very stable substructure. Since the first and last step are always placed there, it would be very soft and the boards would bend.

I would like to, but how and where am I supposed to attach the substructure there? At least 20 cm depth in the first two door niches is insulation. Besides, one should probably not drill into this sealing membrane. The door niche in the third picture is on the old building, directly below is masonry.

Under no circumstances drill. Is there drainage during heavy rain towards the terrace, away from the door? It looks as if the floorboards are lying within the frame and a step is created in front of the door? I would place paving stones (4, 6, 8 or 10cm thick) on the sealing at 3 points, and use rubber pads 10x10cm (1, 3, 5, 10mm) on the paving stones to puzzle together the height and angle of inclination. Cut the step board with a jigsaw and lay it loosely with small wedges. Others would probably level this board with 3 trowels of a mixture, earth-moist, with a rubber hammer.

Drainage? Well, if you mean something like a gutter or a drain - there isn’t anything like that. However, that step there slopes outward and this steel frame doesn’t seal, so water can drain behind it. No step will form: The substructure is built high enough so that the top edge of the board is flush with the steel frame. I would also lay the boards inside the door recess at the same height. Your suggestion sounds simple but effective. Should the boards in the niche be aligned the same way as the others? That would mean I have to fit 6 short pieces in there. Laying the boards widthwise in the door recess would certainly be easier, but turned 90° to the rest would definitely look... problematic.

The substructure is built so high that the top edge of the plank is flush with the steel frame

Then the steel frame is always visible? Then buy an extra wide plank and make the niche from one piece, looks intentional. Or 6 small ones, looks cobbled together. Do not use hollow planks, only solid ones.