in which, of which, at which, to

Prepositions are words that indicate the relationships between various elements within a sentence. Prepositional phrases are groups of words containing prepositions. The preposition, its object, and any modifiers make up the prepositional phrase. A prepositional phrase will begin with a preposition and end with the "object" of the preposition. The pronoun “which” is the object of the preposition. “Which” introduces a relative clause. The use of these prepositional phrases combines two sentences into one. Each prepositional phrase simply refers to what was already mentioned.

I saw a movie. In this movie, the villain goes to jail.
I saw a movie in which the villain goes to jail.
There were ten apples. Four of the apples were rotten.
There were ten apples four of which were rotten.
She showed him the hospital. She was born at that hospital
She showed him the hospital at which she was born.
This is the report. I was referring to this report.
This is the report to which I was referring.