There must be folk who dedicate their time to searching the OSM database for unresolved notes, because whenever I leave a note on StreetComplete mentioning an inaccuracy (e.g: a path where there isn't actually one, a driveway marked incorrectly as a road, etc.) I get an email alert a few days later notifying me that the note has been resolved - often by someone who lives in an entirely different country (and who can use the information I provide in the note plus the satellite imagery to resolve the note).
So thanks for leaving notes allowing remote fixing!
If you want to add an entirely new building on the go, there is Vespucci:
https://f-droid.org/en/packages/de.blau.android/
But, honestly, adding a building on a phone sized screen, while quite possible, is much more easily done with either of the two desktop editors:
JOSM, Java based: https://josm.openstreetmap.de/
iD, Web based, this is the editor available from the "Edit" button from the OSM website: https://www.openstreetmap.org
One thing I wish it had was the ability to add an entirely new building. I'm currently living in a town that is experiencing massive growth and I'll often see a structure that's still a vacant lot on osm, and I don't think you can add it with StreetComplete.