After what felt like an eternity of searching, Alex finally stumbled upon a post that shed some light on the file. It appeared that "X64u.rpf" was indeed a game file, specifically associated with Grand Theft Auto V. It was used by game developers to store data related to the game's 64-bit version on PC (hence the "X64" in its name).
After what seemed like an eternity, Alex launched Grand Theft Auto V with the modified "X64u.rpf" file. The game loaded, and to their relief and excitement, a new feature they had unlocked through the modding process worked flawlessly. It was a small tweak, adding a rarely seen vehicle to the game, but it was enough to validate Alex's efforts. X64u.rpf