While specific versions vary, common functionalities in these scripts often include:
Scripts often use StarterGui:SetCoreGuiEnabled(Enum.CoreGuiType.Backpack, false) to hide the standard Roblox inventory. OP Backpacking GUI
Custom layouts that may allow for more slots than the standard backpack. How They Are Implemented (Scripting Logic) While specific versions vary
How to Make a Custom Backpack GUI | Roblox Scripting Tutorial OP Backpacking GUI