They are fonts that include information how to render those characters, but they also include rendering instructions for normal latin characters and other commonly used characters, because it is quite common to need both. It isn't like wingdings, where latin letters produce icons. Letter A is still rendered as A. You need to write actual Sanskrit characters to add them to your document. So you need to change your keyboard layout to something that produces those characters, copy them from somewhere or use alt-codes.

Computers handle text as list of numbers. On character is produced by sequence of one or multiple numbers in the list. Fonts define how these different numbers should be shown to the user, what kind of lines should appear on screen. So eg. 65 -> A. Unicode is a standard that defines which number means which character. Fonts include instructions to draw only some of the characters, because Unicode includes huge number of really weird characters and emojis. Sanskrit letters have different number codes than latin letters. Keyboard layout defines which number a key press produces.