So 0°F was defined as the freezing temperature of a solution of brine made from a mixture of water, ice, and ammonium chloride (why in the world?). Originally, 90°F was set as human body temperature, which was later changed first to 96°F, and now it's about 98.6°F.

Celsius is just: 0°C is the freezing temperature of water 100°C is the boiling temperature of water

Nobody uses a scale between -18 and 38. People in countries using Celsius just learned as a child that body temperature is 38°C, that's all. -18°C has no special meaning to us.

At 0°C outside it's freezing (32°F). 10°C is quite cool (50°F), you'll need a jacket. 20°C is a comfortable temperature for me, if it's sunny (68°F). 30°C is getting rather warm (86°F). 40°C is hell outside, or a bad fever (104°F). To boil water, heat it to 100°C (212°F).

I get that this seems confusing at first when you're used to completely different orientation points, but for people who are used to C, it's very intuitive.