If you want too steal all your friends loot follow along. First get a hopper and a pickaxe. To make a hopper put a chest in the middel slot in a crafting tabble and suround it with iron ingots(apart from the top slot).

Start mining and go under the chest you want to raid. Break the block under the chest and place a hopper. Now click the hopper and get all the loot inside it.

If you want to clean up the mess, take the hopper and fill in the gap with the block that was originally there!