More specifically, here's how to enter a bench into OSM: https://wiki.openstreetmap.org/wiki/Tag:amenity%3Dbench

You can just enter an amenity:bench poi, but it's useful to include additional information in tags. StreetComplete, for example, has a quest that adds the backrest information.

Osmand lets you add pois, but offers little guidance which tags are appropriate for which kind of poi. Every door seems a bit more helpful in that regard.