Ok, this site says
The preposition is almost always before the noun or pronoun and that is why it is called a preposition
Now, this oxford dictionary says
used after a verb or preposition to mean ‘in, at or to this position or place’
I live here.
Put the box here.
Let's get out of here.
Come over here.
So, the dictionary says "here is an adverb & we can put a preposition before here"
Eg: I am in here
I would say the dictionary is not correct, "here" in "in here" is a pronoun or noun. But I am not sure.
The same thing happened to "there".
Eg: he is in there; he is up there
Can a preposition be before an adverb?