Was there any major changes in the FO universes take on World War 2? Say China defeating Japan in the Second Sino-Japanese War and then annexing Japan after defeating them or did it follow what really happened?

I think the divergence happened after WWII, not during.

Yes, the timelines diverged after WW2.

The Fallout world began after the Second World War ended. So basically all the changes happened after 1945.