It seems to me that everyone has misunderstood your question. What you're really asking is about the meaning of
followed by.
the relative pronoun(whom) is followed by the proper noun(John).
Is it a passive voice?
Yes, it is.
If it is passive then it's active voice: the proper noun(John) follows the relative pronoun(whom).
Yes, that's right.
So, the relative pronoun “whom” is followed by the proper noun “John”, that is " John Whom", am I right?
No.
I am totally confused with the phrase "followed by".
Yes, you are!
We use the verb
follow, in either the active or passive voice to talk about
items in sequence. Let's take the example of a sequence of numbers:
1 2 3 4 5 6 7
The number 5 follows the number 4.
The number 6 follows the number 5.
The number 7 follows the number 6.
The sentences above are all in the active voice. The sentences below are all in the passive voice:
The number 4 is followed by the number 5.
The number 5 is followed by the number 6.
The number 6 is followed by the number 7.
The verb
follow in all of these sentences talks only about the
sequential relation of each number. If we randomise the positions of the numbers, the relation is different:
5 6 2 7 1 4 3
The number 7 follows the number 2.
The number 7 is followed by the number 1.
We can do the same thing if we replace a string of numbers with a string of words.
the woman whom John saw
The word 'John' follows the word 'whom'.
The word 'John' is followed by the word 'saw'.
The verb
follow relates only to the sequential relation,
not the grammatical relation.