The lower point of Fahrenheight is near the freezing point of brine (salt water) which freezes at -6 F (-21 C).
It was designed around what the coldest day at the time of its invention could get and the 100F was marked around how hot the hottest day of the year at the timr would get. Hence its choice to scale 0-100 to local weather vs celcius’ choice to use kelvin and offset it to standardize it to pure water.