What is the subject of the sentence "wherever she lives is where I am destined to be"?

"Where she lives is where I am supposed to be." Here, 'where she lives' is the noun clause that stands for the subject.

Noun clause itself has a subject and verb of its own. That way, its subject is 'she'.

"Where she lives is where I am destined to be." Here, ‘Wherever she lives' is the noun clause functioning as ‘the subject’.

‘She’ is the subject of the clause.

“Where(ver) I am supposed to be” is the complement noun clause.

Wherever she lives is where I am destined to be.

This is a pseudo-cleft construction.

The subject of the sentence is the preposition phrase “wherever she lives” in a fused relative construction. "Wherever" is simultaneously head of the PP and object of "lives in the embedded relative clause". Think of it as meaning "anywhere (that) she lives".

"Where I am destined to be" is also a preposition phrase in a fused relative construction, where the meaning is "the place where I am destined to be", functioning as subjective predicative complement of "is".