The Collins BUBUILD* has this entry, among 12, for "it":
"You use it with some verbs that need a subject or object, although there is no noun that it refers to. Of course, as it turned out, three-fourths of the people in the group were psychiatrists....I like it here....We live in a world in which only the strongest can make it to the top."
So this "it" in your sentence, " How do you like it here?" is a singular pronoun, direct object of the verb "like," representing a kind of amorphous idea. It could mean the ambience, the city, the life, your place in the city, the weather, the people....any or all of those, or other factors.
*The Collins COBUILD English Dictionary. Harper Collins. 1995