Do noun phrases comprise prepositions too? For example:
The apple in the fridge is mine.
Here the noun phrase is "The apple in the fridge" or just "The apple"?
Not exactly -- it would depend on what you mean by "comprise". In your example, "in the fridge" is not a constituent of the noun phrase "the apple in the fridge", but rather modifies "apple". The constituent structure is: [NP the [ apple [PP in [NP the fridge ] ] ] ].
Yes, "in the fridge" is part of the noun phrase in your example because it's dependent on "apple" in this case it's describing it.