It stretches in a few ways, some of the length comes from tunica lengthening, some comes from ligament stretching and some comes from internal penis being pulled out. This is why a good routine should include all of the stretching mentioned.
some stretches can target the inner penis. rotary stretches hit the inner penis. rotary A-stretches and rotary A-stretch blaster hit the inner penis even better. some stretches hit the tunica well like the A-stretch or the blue whale stretch.
Only if you focus your grip low on the shaft. Expressive stretching or stretching internal penis means getting as close to the internal structures as possible to expose and stretch.