Most Japanese descended from Yayoi people. The Yayoi people came from Korea and introduced Japonic languages to the Japanese archipelago. They also married the local indigenous inhabitants. This explains why some Japanese look like Koreans. However, some Japanese display quite strong characteristic of Jomon people. The Jomon people are people who had lived in Japan before the Yayoi people came. Features like pointy nose and bigger eyes that are found among the Japanese came from the Jomon people. East Asians can easily guess if someone is Japanese if the face is very Jomon-like.

However, if the face looks closer to Yayoi type, they can easily be mistaken as a Korean or Northern Chinese. When I was in Kanzai region in Japan, most of the people that I saw look very Yayoi.

As for the comparison between Koreans and Chinese, some Northern Chinese can look like Koreans while other Northern Chinese can look like intermediate between Koreans and Southern Chinese with Southern-like wider nose and bigger eyes. This makes sense because Northern China is neighboring to Korea.

The Southern Chinese look less like Koreans as their faces are rounder/squarer with more tropical features like wider nose and fuller lips.