Short answers, pulled from the story.
Sima Yan declared himself emperor in February 266, ending the Cao Wei dynasty and establishing the Jin dynasty. He forced the final Wei ruler Cao Huan to abdicate and permitted him to live with honor as the Prince of Chenliu.
The collapse of the Western Jin dynasty occurred in 316 when Han-Zhao forces seized Chang'an and executed Emperor Min. This event followed the War of the Eight Princes and the Upheaval of the Five Barbarians which devastated northern China and left the state vulnerable to external threats.
Sima Rui reestablished the Jin government at Jiankang, modern-day Nanjing, in 318 after the fall of Chang'an. This location became the new capital of the Eastern Jin period and marked the beginning of the dynasty's southern phase.
The Eastern Jin force inflicted a devastating defeat on the state of Former Qin at the Battle of Fei River in 383. The Jin army led by Xie An and Xie Xuan exploited internal divisions within the Qin ranks to rout the numerically superior forces of Fu Jian.
General Liu Yu usurped the Jin dynasty in 420 when Sima Dewen abdicated in his favor. Liu Yu declared himself the ruler of the new Song dynasty, which is referred to as the Liu Song dynasty by historians to prevent confusion with the Song dynasty established in 960.
The migration of northern Chinese to southern China caused Southern China to overtake the north in population due to the depopulation of the north. These immigrants were called qiaoren and accounted for one-sixth of the population of the south at the time.