Credits: Amon28
If you're familiar with any of the backpack mods available in Java, you'll quickly grasp how this addon operates. To begin, craft the backpack and hold it in your hand. You'll notice a red indicator appearing next to the icon. While holding the backpack, you won't be able to attack or interact with blocks.
On a PC, simply right-click with the backpack in your main hand to access its inventory. For mobile users, tap the open button or perform a long press on the screen while holding the backpack.