My Korean mom told me that the royalty in Japan were originally Korean. I'm wondering if this is true or just some myth. As an American, I know almost nothing about Asian history, but it would make sense as Korea is the peninsula between mainland China and island Japan so it only makes sense that each migration from China would create a further subset? I mean, all the Asian looking folks must have originated from China and then spread out, gradually settling. Also, why did she specify that it was the royalty in Japan as if to differentiate from the "common" Japanese folk. Very confused and thanks in advance.
Not really. It is a possibility but its not something accepted in history and would go in the category of "we don't know" which means stating it as a fact would be wrong imo. A lot of Japanese early history would go in that category. That claim has been a part of Korean schooling so its understandable why your mother would think that though, its used to undermine Japan and is usually parceled with some more inflammatory claims.
The Japanese Emperor himself has recognized Korean ancestry (which pissed off some in Japan) around the time they joint hosted the world cup but its generally connected to an alliance with a Korean kingdom involving marriage not a royal line that started with Koreans. The Japanese emperor Kammu's mother was descendant of a Korean king Muryŏng. Also while the first bunch of Emperors are considered legendary, the 5th century Japanese emperor Keitai is considered by most historians to have been real (first with that level of historical consensus). Keitai precedes some of the claims of korean origin itself.
Early Japanese history is really messy and uncertain. Here is a thread on it if you want some info on it but again pretty messy. I also really think this post by u/Nelson_Mac is interesting