If you do not have the base game, it is frequently cited by users as available on or through community links on Reddit's r/rugbyunion .
Enter the modders. Hiding in forums like The Rugby Forum (TRF), PlanetRugby, and various Discord servers, a small but passionate group of developers has spent over a decade reverse-engineering the game. They have created a treasure trove of free mods that effectively give you a brand-new game.