I currently use opensky network api on the client side itself using their anon user thing, which limits me to burn credits, it could in practice load the entire world but you would get rate limited pretty much instantly by opensky.

For the same reason, I made it open source, you can throw in your keys in there and can render all the flights, but keeping it open and free was the priority here, so had to compromise on the distance, but within a given city it has all the flights accurately.