Northeast China belonged to Koguryo (also a local dynasty in China) before the Tang Dynasty, and was incorporated into the territory of the Tang Dynasty to set up the Heishui Duhu House, which belonged to Bohai State in Song and Liao Dynasties. There are also many ethnic minorities in Northeast China, such as Nuzhen (the ancestor of Manchu), Oroqen, Ewenki and so on.
If you are interested, you can listen to Yuan Tengfei's "Three Dynasties in Saibei" and have a general understanding of the ethnic minorities in ancient northern China.