EDIT: for clarification: There is only so much space between the front of my head and my brain. How does so much snot fit in there? Is it generated continuously so that there is always more available? Does it come from the lungs, and if so, what is the process of transport to my nose?
https://en.m.wikipedia.org/wiki/Goblet_cell
Blood brings water, sugars and proteins to the cells, the cells make mucus in the mucus membranes. The entire inside of your nose, throat and lungs produces mucus, with varying viscosity and quantity depending on location.