For the benefit of the OP, slightly more ELI5: “by definition” here means that the people who invented the kilogram intentionally said that a kilogram is whatever 1 litre of distilled water weighs (or rather, its mass) at sea level. Basically. So yes, the weight of the soda bottle in kg must be pretty close to it’s volume in litres, assuming it’s filled with mostly water and assuming you’re on the surface of the earth.