How to make a very simple resource pack work with sodium?

I am very very new to making resource packs in Java, I have a large resource/behavior pack in bedrock but I want to port it to Java. So I just want to make a simple one to begin with that adds a "layer" to the iron sword, as far as its been explained to me this layer is not on by default its just like multiple textures for the same "item" that i can give to myself with commands correct?  And if you could also explain if the "pack.mcmeta" file,  "realms" folder, and ".mcassestsroot" file are necessary. Also I dont need to make a crafting recipe for my purposes yet.