OpenStreetMap is incredibly important. So many map applications and project composed from open-data is dependent on OSM (yes, european alternatives to map applications heavily depend on open-data of OpenStreetMap which is actually a database of geographic information, not only the map layer you often see).
I posted awhile ago about Organic Maps which also allows you to edit certain information. But StreetComplete is more fun :)
Microsoft and Apple use OpenStreetMap to an extent. Even TomTom uses it and, probably, Google. The better we make it, though, the less value adds other players can claim and no one can monopolize good street map data.
While I have seen anecdotal evidence for GMaps monitoring chabges in OSM to keep their data up to date, the only "filling in of Gmaps with OSM" I'm aware of is where they used a government dataset that itself was using the OSM license.
That could very well be. I don't have any sources at the moment to prove anything and base it on memory of reading about it somewhere that Both Google and Apple used to more heavily depend on OSM data but over time introduced contractors and more advanced remote-sensing/satellite imagery analysis.