How do custom textured Inventories work on a Spigot server ?

I've been looking into custom GUI and menus, and I know there a lot of plugin that can help you make UI like the one in the photo (MCC Island). But I can't figure out wow you make it ! Like I know it's a resource pack trick but wow do you show that texture to specific inventories ?
Types-of-Cosmetics-In-MCC-Island.jpg
Solution
The general method is to create a font with a retextured unicode character which makes up the shape of the GUI, and then display said character in the GUI title
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