Assalaamu alaykum. Does 7:74, where it says that Thamood were the loins of the people of ‘Aad, mean that they were there descendents, or just that they were their successors?

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, sallallahu ‘alayhi wa sallam, is His slave and Messenger.

The verse does not indicate that the people of Thamood were the descendants of the people of ‘Aad. Rather, it means that Thamood came after the people of ‘Aad (succeeded them).

Ibn Katheer wrote, "Thamood came after ‘Aad, and their dwellings are famous between the Hijaaz (Western Arabia) and Shaam (Greater Syria)... The Messenger of Allah, sallallahu ‘alayhi wa sallam, passed by the area and ruins of Thamood on his way to Tabook in the ninth year of the Hijrah (emigration)." [Tafseer Ibn Katheer, 3/394]

Tafseer Al-Khaazin (2/221) reads, "It means, 'Remember that Allah has made you successors after the people of ‘Aad, whom He destroyed, and has caused you to dwell in their lands to populate them, and you built palaces on their plains..."

Allah knows best.