You need a quest for the way you want to add stairs to. Then when answering you select the other option ("Äh…") different along the way ("entlang des weges unterschiedlich"). Now you can zoom in and split the way. Split it where the first and where the last step of the stairs is. Now you made three ways out of the one. The middle way should be the stairs. A quest should be ready to be answered for the way part to be stairs. Select this quest and go to other answers ("Äh…") and select the option is actually steps ("Das ist eine Treppe"). Now you changed the way part to be stairs.

ah yeah, this is the Things overlay

Yes, this is one way. But you can use StreetComplete's overlay feature (since release https://github.com/streetcomplete/StreetComplete/releases/tag/v57.0) to directly add benches and trash cans. This helps in avoiding to flood the note system with very small map features. When the overlay feature was not implemented i also added a lot of notes for benches/bins, but got a lot attention of other mappers because of this. So i switched to using OSMTracker to take notes. But now, i am a very happy user of the new things overlay.

Adding stairs is also possible, but a bit more complicated, so adding a note is fine.

Do you know this overlay feature already? This is definitely worth to check out.

I have got a second one mainly for mapping, because i want to use a stylus during cold winter (with gloves) and hot summer with sweaty fingers. I hate touch inputs and with the stylus it works easier. This phone also has an SD card slot for all the different map apps, their map data and the OsmTracker photos.