All custom NPC modifications are stored in a container file with the following data.
When the game starts, it will load all the custom models inside the StreamingAssets folder. You will be able to see the mod(s) data inside the Custom Content menu. Here you will also be able to select which mod you want to apply to any of the game characters.
To see the loaded mods, go to the Custom Content menu.
The Custom Content menu will list all the loaded mods. The dropdown on the top-right lists all the moddable characters.
When you select a character, click on a mod, and the character will re-spawn with the mod settings, in this case, a new model.