Is it necessary for the intention of purification from Janabah (menstruation, sexual intercourse, wet dream) to be made upon entering the bathroom and is that sufficient, or upon completing the regular ghusl? And is one intention sufficient for purification from all forms of Janabah, such as menstruation and wet dream together?

The intention (niyyah (definition)) should be made at the beginning of the ghusl (definition) (ritual bath), and it is permissible to make it a short time before. A single ghusl is sufficient for multiple causes if the person performing it intends all of them. Scholars have differed on whether the intention for one cause suffices for another if only one is intended without the other, and some have favored its sufficiency.