The PC mod enables you to download and play any number of modes and specially created maps directly from your Rocket League titled screen https://www.lolga.com/rocket-league/trading-prices. You can even play these modes online, though it is a little complicated to do since custom mode matches aren’t hosted on official Rocket League matchmaking.
This particular Air Racing mode was first released in August 2020. Since that time, Air Racing has built a small yet devoted community https://www.lolga.com. The creators had a very specific competitive intent from the very start, and they have organized their players via Discord and YouTube.