I won't go that far as to say that Japanese are descendants of Chinese although there might some DNA in some individuals.
(Korea will have a stronger DNA linkage than Japan which is separated from the Asian continent by Pacific.)
Let me explain, when China was culturally light years ahead of the neighbouring territories, the residents of these territories were probably still running around with no pants on, particularly in S. E. Asia. That is why China, in Chinese language is the "middle kingdom" - i.e. centre of the world, the rest of the world were just "peripherals".
The privileged class of the neighbouring territories considered it an honour/status symbol to be able to read and write perfect Chinese. That is why there were a lot of Chinese words being used for names of places and persons even up to this day.
Starting in 1950, both Japan and Korea were making a conscious effort to move away from using Chinese words for place and person names. So, these days we don't see too many Chinese words being used in Japan and in particular, Korea.