Files From PC to Steam Deck (Abbreviated to SD) Using HDD

So, question when it comes to transferring files from PC to SD. I am using an external, would it replace any files that are missing or aren't linux files or would it reinstall the whole game? Would it be better to install the game on the SD? I have a data cap so I would rather want to transfer and have to only install some files and not the whole game. Thanks! How could I go about it?