Do the "seventy men" mentioned in the verse "And Moses chose from his people seventy men" represent all of Moses' people, or did Moses choose them from among his people? And how were they chosen? And did all of them fast for forty days like Moses? And was the reason for choosing seventy men specifically mentioned in the hadiths or books of tafsir?

Moses (peace be upon him) chose seventy men from his people to apologize for their worship of the Calf, even though his people were far more numerous, consisting of twelve tribes. As for Allah's appointed time (miqat), it has no relation to the timing when Moses was spoken to by Allah and given the Torah. In Surat Al-A'raf, Allah mentions His promise to Moses and giving him the Torah, then mentions his people's worship of the Calf, then Moses's return, then Allah's warning to the calf-worshippers, and only after that were the seventy chosen.