Making an alternate texture for an item

I saw this video about custom model data that showed a guy making a carrot on a stick have different textures, but all at the same time. I downloaded the pack in the description and made a couple of modifications, basically just changing the item from carrot on a stick to diamond sword, just because I don't know enough about datapacks to give a carrot on a stick a damage modifier or make just a custom item or anything. Any help would be very much appreciated. Thanks!

video: https://www.youtube.com/watch?v=tvL9Z0blPC8