I see many games with near exact inventory systems as Roblox’s, but the only difference is that they look like they have customized UI instead of the default Roblox inventory, while acting the same as Roblox’s default inventory. Are these games recreating the inventory from scratch or do they simply do something to customize the already implemented inventory system?
I believe these are custom implementations. Roblox’s inventory system isn’t super complex, it just moves items from a backpack to a player model, so the hard part would likely just be making the UI
You cannot customize Roblox’s inventory system, you have to make your own. Another benefit to having custom inventories is not being required to use tools for items, and allowing more text on items for values like quantity, durability, etc…
TL;DR: Yes, you do have to create your own inventory system.