If you are looking for a "paper" (technical guide or documentation) on how these trainers function, they typically work by injecting code into the game's memory (RAM) to modify specific values while the application is running. FLiNG Trainer - PC Game Cheats and Mods

Ubisoft has automated ban waves. Because a "Fling Trainer" requires memory writing, it triggers the same signature as a money hack. A ban in The Crew often extends to Rainbow Six Siege or other Ubisoft titles using the same anti-cheat.