There are two different ways the strings produce spin:
1. Ball to string friction: Textured strings will bite the ball and apply more force to the ball, producing more spin. Smooth strings do apply spin, but they have less ball to string friction. The rougher the string (or more sharp angled), the more friction = more spin.
2. Having a low string to string coefficient of friction: When the strings are really smooth and slide over each other well, then you get increased string "snap back" which applies force to the ball to induce spin. If the strings are very rough (such as Tecnifibre X-1 Biphase), then they will be "locked together" and not slide over each other. You will not gain any additional spin since there is no string snap-back.
And lastly, there's the obvious #3: Technique. Swing path, tip speed, racquet face, acceleration on contact, etc. Those produce spin as well.
The best way to maximize topspin is to have a little of all three things. You want good ball to string friction, low inter-string coefficient of friction, and good technique.
