Learn Finnish: Miksi kellarissa on niin paljon roinaa? - Why is there so much junk in the basement?

You would use se, mitä (that which / what) because we are replacing a partitive mass noun phrase. The emphasized part moves to the end to become the focus: Se, mitä kellarissa on niin paljon, on roinaa. (What there is so much of in the basement is junk.)

Why does the sentence start with the location (kellarissa) instead of the subject?

This is a classic existential sentence, which introduces new information by stating that something exists in a certain place. In Finnish, these naturally follow a Location + Verb + Subject word order. If you started with the subject (Roina on kellarissa), it would mean 'The junk is in the basement,' implying the listener already knows about this specific junk.

Could we drop niin paljon and still use the partitive roinaa?

Yes. Even without niin paljon (so much), you would say Miksi kellarissa on roinaa? because roina is an uncountable mass noun. In existential sentences, mass nouns acting as the subject take the partitive case to show there is an undefined or partial amount of it.

What kind of 'junk' does roina refer to, compared to words like roska or romu?