Learn Russian: Вот квартира, в которой находится большая ванная. - Here is the apartment in which a large bathroom is located.

The word которой refers back to квартира (apartment), which is a feminine noun. Because we want to say "in which" (inside the apartment), we use the preposition в followed by the Prepositional case. The feminine Prepositional form of который is которой.

Why use the verb находится here? Couldn't we just say в которой большая ванная?

You could just say в которой большая ванная or use есть, but находиться (to be located / situated) is very common when talking about the physical location of rooms, buildings, or geographical places. It makes the sentence sound a bit more formal or descriptive.

Why does the word for bathroom, ванная, have an adjective ending?

Ванная is actually an adjective that turned into a noun! It comes from the phrase ванная комната (bath room). Over time, Russian speakers dropped the word комната, so ванная now stands alone to mean "bathroom." Even though it acts like a noun in a sentence, it still takes feminine adjective endings when it changes case.

Why is the verb placed before the subject in the second half of the sentence (находится большая ванная)?