Learn Italian: Le porte delle chiese sono chiuse. - The doors of the churches are closed.

Delle is an articulated preposition. It is a mandatory combination of the preposition di (of) and the definite article le (the). Because chiese (churches) is feminine plural, it requires the article le, which merges with di to become delle.

Could I just say di le chiese instead?

No, in Italian, when the preposition di is followed by a definite article like le, they must combine into a single word. Leaving them separate as di le is grammatically incorrect.

Why do we use di (inside delle) here rather than da?

The preposition di is used to show belonging, possession, or a relationship between two things, translating perfectly to "of" in "the doors of the churches". The preposition da generally means "from" or "by", which would not fit the meaning of this sentence.

Why does the word for closed (chiuse) end in an -e?

Adjectives must match the gender and number of the noun they describe. Here, chiuse describes le porte (the doors). Because porta is a feminine noun and it is plural here, the adjective changes from its default dictionary form (chiuso) to the feminine plural form (chiuse).