Is it obligatory for the water to reach the hair itself during ghusl, or is it sufficient for it to reach the skin beneath it, and does the presence of dirt that prevents water from reaching some hair on the testicles affect the validity of ghusl, especially when purifying from madhy, which requires washing the penis and testicles?

In ghusl (definition) from janabah (definition), it is obligatory to run water through the hair of the body and ensure water reaches the skin beneath it. As for dirt originating from the body, such as dried sweat, ghusl is sufficient even with its presence. Regarding madhy (pre-seminal fluid), the preponderant opinion is that it is sufficient to wash only the area that has been affected by the impurity.