How does the computer know to only use the arrow keys and not just any button on the keyboard?

This is largely determined by the script attached to the object that you intend to move with the arrow keys. In the script you would tell the object to move in the direction that you want it to move whenever a specific key is pressed. In this project we used the word "Horizontal" and "Vertical" to determine what keys to gather information from. The word "Horizontal" gathers information from the left and right keys and the word "Vertical" gather information from the up and down keys. Once we have this value we can assign it to the game object to move in the direction that the key is pressed. Unity assigns the word horizontal and vertical to the specific keys by default and you can see this in the Input panel located in the Project Settings option of the Edit Menu.