As an example, this will use Reinforced Puffer Vest (Items.Vest_17_basic_01)
The base ID
An item's spawn code is called BaseID. It looks like this:
and will be used to spawn items via CET (Game.AddToInventory("Items.Vest_17_basic_01")).
A base ID consists out of two parts. The first one (Vest_17) determines the item, the second one (basic_01) determines the variant.
Finding the files
Head to WolvenKit and search for the first part of the BaseID:
Search WolvenKit for the first part of the BaseID
You will find at least one entity file and an appearance file.
If you find multiple files, you need to open the correct one. You can identify those by the folder path: for player meshes, the name of the last folder will contain either _pwa_ or _pma_, and the path will include either that or player.
The .app
Open the apperance file: this is where the item and its variants are actually defined.