Using code, how would I add onto a pre-existing vanilla Minecraft GUI? (The Inventory)

For my mod, I'm trying to add onto the player's inventory. Basically, if they are in the inventory, they will see another tab that they can click, and it takes them to a different part of the inventory. (Imagine the PlayerEx mod, that allows you to access the attributes tab from the inventory.)

Of course, I'm well aware this requires code, and I'm perfectly fine with that. I don't know how to code, but if someone could help me with this problem, that would be great. I'm not even sure where to get started. I found the Inventory code in base vanilla Minecraft, but not sure what to do with that.

I realized that the Overlays pretty much have the ability to do this, but not GUI? I'm sure there's a way to transfer the Overlay's ability to do this over to the GUI, but I'm unsure of how to do so. I tried to mess with the code, but I'm terrible at it and couldn't make any sense of it.