I did some research for a discussion on another website and I thought that I don't want to waste all that effort it so will post it here. Infer what you will.

The Biggest Vt*ber is Gura(I assume) at 3.93 million subscribers, she is American.
Kizunai AI has 3.09 Million subscribers she is Japanese but she has retired.
The biggest Japanese Hololiver and so the biggest living Japanese Vt*ber(I assume) is Pekora at 1.92 million subscribers.

No sources here but they aren't hard to look up.

The biggest Japanese Youtuber is Junya at 15.3 million subscribers. He does weird things.
The Biggest Japanese Gaming Channel is Hikakin games at 5.55 million subscribers.

https://socialblade.com/youtube/top/country/jp/mostsubscribed

Something to note about this list is that it goes by the country that Youtube says they reside in not who watches them or what language they use. So number 10 is a Slavic girl making videos in some kind of Slavic language and the comments for her videos are all written in Cyrillic. Gura is also listed here too.

Now Moving on to Japanese Twitch. The biggest Japanese Twitch channel is FPS_SHAKA at 822k subs.
The Biggest Vt*ber is at number 12 and is sakuramiko_hololive at 243k subs.

https://twitchtracker.com/channels/most-followers/japanese


It's important to note that perhaps up to half or even more in the case of popular Vt*bers are most likely foreign(there was a clip of two of them showing Youtube analtics that showed somewhere from 30-50% were foreign and they were smaller Vt*bers).