Comparative adverb

I was taught that object of a preposition is always a noun, but I have often seen that a comparison adverb comes immediately after a preposition, then a noun phrase preceded by an adverb comes, which is beyond my grammar.

Note: Please don’t tell me its meaning, I know it very well, rather it is all about grammar.

He has been here for more than 2 years.

After "for" more than comes immediately, which is an adverb and is modifying the noun somehow.

Could anyone tell me what is the function of more than here and are they two combined words or separate?
If they are separate, what are they doing individually in a gramatical sense?

Just like that, schedule me for as soon as possible.

Here again we have comparative adverb after "for".