So there are few reason why someone wants to find their fortnite games files
Copying fortnite game files
The first one and the most common one is to copy those files and paste them to another computer without having to download the game.
Editing Fortnite Config File
The second most reason why people access their fortnite files is to mess with their config file because for exemple you can fix your in game resolution or get a stretched resolution maybe,or aslo making sure that your fortnite settings are saving.
Access Fortnite Game Files
First press your windows Key and write %APPDATA% and open the file.
now once the folder pops up go to back to app data (it’s before Roaming on the top)
Then go to Local
and then you will find the fortnite files folder
If you want to access the fortnite config file go to FortniteGame > Saved > Config > Windows Client and you will find the file you’re searching for its GameUserSettings.