I think with StreetComplete one has some implicit expectation that it's being used while walking around live. But you can also use it for normal editing of areas you have been to (recently enough to say that there is a sidewalk). And getting points for that motivated me to look at all places where I know for sure about sidewalks, etc, because I've been there ;) This might be similar for other people. But you are right, streetcomplete sets source=survey. Some people might use other sources.

Yes. Not on the same day obviously, but I walked these places.