All perfect praise be to Allah, The Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
The water must reach all parts of the body during Ghusl, and it must also reach the skin of the body parts that must be washed in ablution. If the water does not reach the skin underneath the hair except by running one's fingers through the hair, it is required to do so in order to make sure that water reaches the skin underneath the hair, regardless of whether the hair is long or short.
If there is no need to run one's fingers through the hair and it is most likely that water will reach the skin underneath the hair, then that is sufficient in the obligatory Ghusl or ablution, and the state of ritual impurity is thus ended properly.
Allah knows best.