among当中 means "among, in the midst of" — picking out a position within a group or space. With a group it's "of all …"; it can also be physically "in the centre," or "in the middle of (a process)."
Structure: Group / Place / Process + 当中
you你are是my我all所有friends朋友of当中the most最clever聪明的。
Of all my friends, you're the cleverest.
them他们among当中many很多都are是Shanghainese上海人。
Many of them are Shanghainese.
Remember: among当中 = "in the midst of" — of a group ("of all…": friend朋友among当中), a space ("in the centre": room房间among当中), or a process ("mid-…": to negotiate谈判among当中). It always sits after what it's inside.
Watch out
- It comes after the group/place, not before: friend朋友among当中 ✓, not among当中friend朋友.