One of the "basic" principles of thermodynamics is that, for gases, there is a direct link between temperature and velocity. Namely, if the pressure remains constant, temperature and velocity are inversely proportional: if you reduce the velocity, temperature raises; if you increase the velocity, temperature drops. When your mouth is fully opened, air comes out slower, so it is hotter ; when you "fffuuuuu", air comes out faster, so it is colder. There are other applications of the Venturi effect, including circumstances where the "constant pressure" approximation is not valid anymore.
I actually said "WHAT THE FUCK" out loud lmao
It's about the air pressure decreasing in a smaller bottleneck