Making and referring from text files

So I'm making a game and I'm trying to include a system where an intro plays when the game is loaded up, but the intro will only ever play once. Based on other big games I've played, I know they may often refer from a notepad file. For example, in FNAF 6, you can increase the monetary value, and after a reload, the value is the number you put in (I'm sorry, I'm going off on a tangent. Anyway, I have not a clue where to start, so any advice would be appreciated!

 

You need a way to save your game - you can either use Unity's method or find some useful tools in the Unity Asset Store that make saving your game easy.

Next you can make the intro part of your save system

If the save file exists - skip the intro - because the player has played before.
If the save file does not exist - this is the first time the player is playing the game, then play the intro and save something to create the save file.

Or alternatively set a variable when you have played the intro, and test that variable at start.

 

Look up PlayerPrefs, it's the basic system for saving and loading values in Unity. No need to muck around with text files.