I'm not an expert, but if you press the layer button next to the three stripes on the top right corner, you can choose between different map layers - the second to last option has a green dot icon and is probably called something like "objects" (I'm using the German version, so I don't know what the option is called in English). Then, after choosing a spot and pressing the plus button on the bottom left, you can add various objects like benches, trash cans, trees etc. yourself without having to leave a note for someone else to add it. But I think StreetComplete can only add little objects and metadata to already existing things, so for creating completely new streets, stairs, buildings etc. you have to use another OSM editor.