They are mostly chosen by common sense in terms of accuracy. A persons weight is in kilograms because it usually doesn’t matter if you weigh 75 or 75,3 kg so there is little need for 753 decagram to express 75,3 kilogram, but if it is important, you can use 75.300 grams. In recepies grams, kilograms, milliliters, centiliters, deciliters and liters are common but you’ll still find teaspoons, tablespoons and in Dutch even remnants of the pre-SI era like ‘ons’ (100 gram) reminds a bit of ‘ounce’.