ᐅ Close gap between staircase and wall

“Favorite parts” because they are labor-intensive to customize, right?
Alright, then at least I now know what I need to focus on.
Thank you

There are also suitable parts available at hardware stores, but I actually still see the stair builder as responsible here. To me, this is part of a proper professional installation.

Basic question: Why isn’t the stringer placed directly against the wall?
Why is there a recess?

Solution: Possibly leave it as a "reveal joint" and only paint the visible lower strip (concrete?) in the wall or stair color (definitely tricky).
This way, varying widths are less noticeable to the human eye.
Cleaning would also be a challenge.

Favorite parts for carpenters because they can hide everything behind moldings. When someone announces moldings everywhere, I have my own thoughts.

Did you have the stairs or plastering done by an external company?
In our case, we have the plastering done by a separate company, but the stairs are installed by the main contractor (soundproof stairs, so with a gap from the wall), and plastering is done only afterwards.
In our situation, we have to take care of covering the gap ourselves. Maybe it’s the same for you?