定语从句中表语中by+which的问题？

这里的by是使用、依靠的意思。
把句子拆分成两个句子你就理解了。
1.the mechanism is not clearly understood.
2.brain cells store memories by mechanism.
然后合成一个句子当然就要对mechanism使用定语从句来修饰。
the mechanism （by which brain cells store memories） is not clearly understood
理解了吧。

The mechanism( by which brain cells store memories) is not clearly understood!
也可以这样写
the mechanism ( where brain cells store memories ) is not clearly understood
也等于
the mechanism (which brain cells store memories by) is not clearly understood

by which = where

by表示被动。 因为后面出现is not clearly understood
还原的话它是放在the mechanism的后面。