The "120 FPS file" is a modified configuration file that tricks the game into thinking your device is a high-end iPad Pro or a flagship Snapdragon 8 Gen 2 phone. By replacing your original config file with this modded one, you force the game to unlock the hidden "Extreme+" or "120 fps" option in the Graphics settings.
PUBG Mobile stores its graphics settings in configuration files (usually with .ini or .sav extensions) located within the game’s data folder on your internal storage. These files contain a list of "supported devices" and their respective maximum FPS caps.
PUBG Mobile is a game where every millisecond counts. With the recent version 3.2 update, the developers officially introduced 120 FPS support for high-end devices. However, many players with capable hardware find this option locked in their settings.