Cladding concrete stairs with tiles, vinyl, or parquet?

The commissioning of the concrete stair builder is done by the general contractor (GU). I am also planning several inspection points by an independent expert. One of the inspection points would be after the shell construction (including the stairs).

I was only thinking about commissioning a craftsman myself for things like flooring and stair covering, since the person working at the GU only has very limited options.

That seems like a clean boundary (at least from my perspective :) ) and does not require extensive coordination with the GU craftsmen.

Another thing I wanted to do partly myself are some things related to smart home, but I am still discussing a clear delimitation of responsibilities with the GU electrician.

Does that make the plan better?

No, unfortunately it does not. My remark

No. Your stair builder is already coming to the construction site when the GU’s work is not yet ready for acceptance.

applies not only to a stair builder in the sense that he builds the entire stairs outside the GU’s contract volume, but also in essence to one who "merely" covers the shell stairs built by the GU or the floors (adjacent above and below). Here he even has the risk of the subsequent trade. I do not share your perspective of a clean boundary at all, rather I clearly categorize it as “wishful thinking.” These are exactly the fiascos or, if you prefer, failures that completely surprise laypeople as to how they could thrive so splendidly with such a seemingly small risk. The yellow-black striped barrier tape at the boundary of the GU’s special request horizon warns of the abyss behind it and is not to be understood as an invitation to jump over it with presumed courage. If a 95% match between dream house and actual offer is not enough for you, then you are in the wrong place with the GU. Covering this warning light and going headfirst through the wall will bring the pedal boat full speed ahead into distress. What lurks behind this red line is being minced as material for a scripted reality construction botch documentary. Many builders forget that, due to the value in dispute, you practically always start your disputes in house construction at the regional court. Also have a clean interface set for your smart home ambitions, i.e. the electrician only pulls cables and installs the sensors and actuators, and in the larger selected control cabinet there is a PLC which the electrician does not need to understand anything about. For everything else, I recommend the multiple threads here on the topics of smart home / bus systems / LAN / access points.