How do nylon and polypropylene blend together in the production of a toothbrush?
A toothbrush need not be made of the two materials you mention.  The two materials are choosen because of their low price of production*), ease of manufacture of the intended tooth brush (heat molding of the frame), moderate / tuneable flexibility of the material for the bristles (nylon).  In comparsion to bristles of natural origin, nylon equally is easier to cleanse by rinsing with water, and dries rapidly ‒ offering less ground for bacteria to grow on them.

*) Waste disposal is not accounted here. While the brush, consisting of polymers, may be incarcerated, why do we throw away the whole toothbrush away if apparently only the heads / bristles wear out?