What is the post-menstrual ritual bath (ghusl al-hayd)? And is it forbidden for one who has had intercourse with his wife to eat, drink, or mix with people before performing the ritual bath?

The jurists did not speak of something called "the great ghusl," but it appears that the questioner is asking about the method of ghusl after menstruation. This does not differ from the ghusl of janabah, except that it is recommended for the menstruating woman to trace the blood’s trace with some perfume. The correct view is that the menstruating woman is not required to undo her braids during ghusl, unlike the one in a state of janabah. What the questioner mentioned about the prohibition of eating, drinking, and speaking for the one in a state of janabah before ghusl is incorrect, for the one in a state of janabah is pure, and nothing is forbidden to them except what the Sharia has indicated as forbidden, such as prayer and touching the Mus'haf. It is recommended for the one in a state of janabah to perform ablution if they wish to eat, drink, or sleep.