Balloon is filled with normal saline inside the bladder. A standard Foley is a double lumened tube, one with a balloon on the end and the other just ending in nothing. You fill the balloon once you insert it, then it holds it in the bladder, empty it when you want to remove it. Then on the open end you attach the bag and it’s held on with friction. Really though a sudden yank may pull the balloon and that’s… Less than ideal.