who引导定从的用法,some of whom还是some of who?It's the same in China—

who引导定从的用法,
some of whom还是some of who?
It's the same in China—many people,some of whom are not overweight at all.
可以看到,这个whom在从句中作主语,但为什么要用whom而不用who呢?说是有of这个介词在前面,那么我想问,当who引导定语从句时,他的宾格whom一种是在从句中作宾语时用(此时必用whom吗?）,在一种就是即使who在从句中作主语,但前面有介词,所以用whom来作从句中的主语,