If you drink a cup of water or eat something and immediately step on the scale, yes, you'll weigh that much more because the liquid/food is in your stomach.
As the food is digested, the various nutrients are metabolized throughout your body. Water is not only passed through your kidneys into your bladder, it's passed into your bloodstream, into your muscles, into your skin, into all the tissues of your body.
You lose that water by breathing (yes, you lose moisture with each breath), by pee'ing, by sweating, and just by evaporation from your skin.
If you don't replenish that water, then you become deyhdrated.