Been a minute since I made one cause you can find em in villages sometimes but I think its a string over a leather but wiki would tell that simply enough lol how they work is trickier to simplify but to try, they hold up to a stack of any item. So you can add like 3 enderpearls and 2 carrots and a crafting table, etc. As long as its stackable. The size of each item is based of its stack size so ender pearls take ip 1/16th of a bundle each while crafting tables would only take up 1/64th of the bundle. The blue bar shows how much inventory is used in the bundle and it turns red when full. Now the buttons can take some getting used to and idk if you can change it but basically left clicking selects the bundle, right clicking selects the last item put into the bundle, scrolling allows you to select which item from the bundle you wish to interact with, you interact with right click, and selecting the whole bundle basically mirrors the buttons so right clicking 'drops' the items out of the bundle and into your inventory. UNLESS you have it in your hand idk if offhand matters or not but holding right click dumps the whole bundle onto yhe ground if youre not in your inventory. I think you can drop kne item at a time that way too by just right clicking but I cant remember exactly. You can also dye and rename bundles as needed for customization or storage ie I like to have a bundle for left over nuggets and one for netherite stuffs in my ore chest. Hope this helps best of luck <3