That data is not available through ADS-B, and hence not in the API. The flights are joined with our
opensky-network.org/aircraft-database, which is free to download. There are also much more extensive community tools, such as
traffic-viz.github.io/data.html which integrate both.
Re: coverage: Indeed, I think everyone is well aware, after all it's visible on the main explorer site and in detail on
opensky-network.org/network/facts
Rule is: If you want coverage in your area, set up a receiver! If you know someone in a place that's not covered yet, have them set up a receiver. It's fun and not expensive.