Creating bottomless buckets

First, you need to get 5 gold blocks, a nether star, and whatever bucket you want to make bottomless, dig a hole in the ground and place the gold blocks like this, or just use support blocks, and throw the bucket in the middle of the structure and while holding the nether star, and aiming to the inside of the structure, try to mine any of the gold blocks, you can just mine the gold block in the middle to make things easier

If done correctly the gold block will disappear, you´ll consume one nether star, and the bucket will become bottomless, if you are close enough you´ll get it on your inventory, if not it will shoot upwards a few blocks and fall back again

Using bottomless buckets

These buckets work the same as they would when using them in creative, for the lava, water, and powder snow you can place as many blocks of them as you want, powder snow is especially useful due to it allowing you to make bridges and towers you can climb if you have leather boots, as for milk, you simply can drink it infinitely to get rid of any nasty poison effects