Unfamiliar wording:
1 – “When he performed ghusl after janābah”: that is, when he intended to do so. Al-Zamakhsharī explains that the text expresses intention by using the verb itself, because an act comes about through the doer’s capacity and will; the wording is concise for brevity’s sake.
2 – “He would then run his fingers through his hair”: takhlīl is to insert the fingers between the strands of the hair.
3 – “Until he had moistened his skin”: meaning that he conveyed the water to the roots of the hair. The skin meant here is the visible surface of the scalp covered by hair.
4 – “When he thought”: here ẓann denotes a prevailing assumption; there is no proof that absolute certainty is required, and acting on probability is accepted in legal rulings.
5 – “He poured water over it”: i.e. he let the water flow over his hair.

Overall meaning
ʿĀʾishah describes the Prophet’s ﷺ manner of bathing after janābah. Whenever he wished to take a full bath, he would begin by washing his hands so that they were clean when scooping water for purification, then make ablution just as he would for prayer. Because of his thick hair, he would run his wetted fingers through it until the water reached the roots and saturated the scalp. He would then pour water over his head three times and wash the remainder of his body. Despite this thorough washing, one vessel sufficed for both him and ʿĀʾishah, each of them drawing water from it in turn.

Lessons derived from the hadith
1 – The ghusl after janābah is prescribed, whether the cause is ejaculation or mere penetration, as will be stated explicitly in the hadith of Abū Hurayrah.
2 – A complete ghusl is as described here: first washing the hands, then performing ablution, then finger-combing and soaking thick hair, followed by washing the rest of the body.
3 – Her words “Whenever he performed ghusl” indicate that this was his consistent practice when bathing after janābah.
4 – It is permissible for either spouse to look at the private parts of the other, and for them to bathe from a single vessel.
5 – The limbs washed in ablution are to be washed at the beginning of the ghusl, except for the feet, which are delayed until after the entire body has been washed, as will be explained.