Student or Learner
1.(a)He is coming from London.
..(b)Where is he coming from?
2.(a)He went to London.
..(b)Where did he go to?
I think here 'where' is the object of preposition.Therefore,
.........(1)Is 'where' considered as pronoun or noun here?
.........(2)What are the other preposition used like 'where...from?,where...to?'?
That's usually called an an adverb.
Collins calls 'where' in "Where are you going?" an adverb.
Random House calls it a pronoun in "Where did you come from?" (Both on above page).
It could be the object of a preposition if it's a pronoun. But there's no preposition in 1.
It's only rarely a noun - as you can read above.