It... isn't. That would change wildly depending on which sea/ocean you get your saltwater from (more salt = colder freezing point).

It really is defined relative to a very specific brine mixture (in the most scientifically generous origin story - some say he literally just measured the coldest winter day he could). Well except it isn't anyway, because like all US units nowadays it's defined against metric units (namely the Kelvin, just like 0°C is actually defined to be 273.15 K).

There is no freezing point of salt water. Cause water can have a very small or very large amount of salt in it. There isn't even a "default" amount of salt that's just assumed.