Where does the total 32 coming from? I do understand the concept of freezing water at 0° C and 32° F. But how can we give a definition for "32°F" to understand how and who determined it? Where did it come from?

The main reason why freezing water is 32 on the Fahrenheit scale is because he (Fahrenheit) devised his original scale based on something other than the freezing and boiling points of water. Instead he chose to set 0 as the freezing point of a particular brine mix consisting of ice, water, and ammonium chloride and 96 as normal body temperature. This put the temp of freezing water close to 32, but not exactly. Later he refined his scale - he got rid of the brine mix idea and instead defined freezing water as exactly 32 and body temperature as 96. This put the temperature of boiling water close to 212, but not exactly. Still later after he died his scale was modified again to define freezing water as 32 and boiling water as exactly 212. So now F is based on the same physical phenomena as the celsius scale, but with a total of 180 degrees between them. This has the advantage of making it easy to convert C to F, using the formula F = Cx9/5 + 32. Normal body temperature is now 98.6 degrees F, not 96.