It seems to use jawg.io for its base map. Perhaps it’s this?

It brings me a lot of joy to contribute to OSM. I haven’t written anything other than the occasional script for years, so it feels good to help the community in ways like this instead.

I’ve sunk so many hours into it. Either “armchair mapping” with OSM and the imagery overlays, or (more fun) wandering with the android Street Complete app to fill in gaps when I’m out and about.