Are all the people surnamed Jiang the descendants of Jiang Ziya?

not necessarily, but it is possible. The basis is as follows: It may be the basis of Jiang Taigong: In the pre-Qin period, the areas where Jiang lived were mainly Gansu and Shandong. Emperor Yan was born in Jiang Shui, and his descendant Jiang Ziya was sealed in Qi at the beginning of the Zhou Dynasty. By the middle of the Warring States period, he was destroyed by Tian's family, and his descendants were scattered. Some people took the country name as their surname, or took the surname as their surname.

Not necessarily: During the Tang and Song Dynasties, the surname Jiang entered Fujian and Guangdong. Huan Tingchang, Minister of the Tang Dynasty, changed Jiang's surname.

In the Qing Dynasty, due to the migration from Shandong, the comprehensive sinicization of the Eight Banners of Manchuria and the use of Han surnames, Jiang surnames in Northeast China developed rapidly. All Jiang Jia clans and some Zhang Jia clans in Manchu Eight Banners in Qing Dynasty collectively changed their surnames to Jiang.